0:00:44 > 0:00:47I'm on the holy mountain of Croagh Patrick,
0:00:47 > 0:00:51where St Patrick is said to have fasted for 40 days.
0:00:51 > 0:00:53Once a year, thousands of pilgrims make the climb
0:00:53 > 0:00:56to the 762-metre summit.
0:00:56 > 0:00:59Many of them in bare feet.
0:00:59 > 0:01:04Some Catholics brave the pain of this barefoot pilgrimage as a penance.
0:01:04 > 0:01:08But I'm here on a mission of my own.
0:01:08 > 0:01:12The pilgrimage I'm making is to celebrate one of nature's great spectacles,
0:01:12 > 0:01:16and you need to get high up to take it in.
0:01:16 > 0:01:19The extraordinary islands of Clew Bay.
0:01:33 > 0:01:38It's a beguiling water world, unlike anything else in the British Isles.
0:01:38 > 0:01:44Local mythology counts Clew Bay's islands at 365.
0:01:44 > 0:01:48One for every day of the year.
0:01:48 > 0:01:54I'm intrigued to discover how this community of islands once supported a community of people.
0:02:00 > 0:02:04Mary Gavin-Hughes still sails these waters.
0:02:04 > 0:02:10She's one of the last generation of self-sufficient islanders who've fished, and farmed, in Clew Bay.
0:02:10 > 0:02:12What was it like living on the Islands?
0:02:13 > 0:02:16It was heaven on Earth living on the island.
0:02:16 > 0:02:19It was very peaceful, great tranquillity.
0:02:21 > 0:02:29Mary grew up in a world with no electricity, in a tight-knit community separated by water.
0:02:29 > 0:02:30What's that building over there?
0:02:30 > 0:02:35This one here is, erm, known as Cullen school, that's Cullen Island, that was the school.
0:02:35 > 0:02:37- That little white building?- Yep.
0:02:37 > 0:02:40That's the smallest school I've ever seen in my life!
0:02:40 > 0:02:48By the time Mary was a teenager, she was roving around Clew Bay on her own.
0:02:48 > 0:02:52This picture here shows how we used to row to and from home.
0:02:52 > 0:02:56Here you are. It's a heavy looking boat, these oars are absolutely huge.
0:02:56 > 0:02:59They were handmade, my dad actually made them, and, erm...
0:02:59 > 0:03:02Yeah, they were good and sturdy, but we needed them
0:03:02 > 0:03:05for the weather we were up against sometimes.
0:03:05 > 0:03:09- You look as though you're enjoying yourself there. - Of course I am.
0:03:09 > 0:03:10Smile, Charlie.
0:03:12 > 0:03:14That's his home.
0:03:16 > 0:03:22Mary's father taught her to feel at home on the water, harvesting the sea's bounty.
0:03:22 > 0:03:25But they didn't live on fish alone.
0:03:25 > 0:03:31We did all our farming on the island, our fishing, and we were very self-sufficient.
0:03:31 > 0:03:34The grass seems really quite lush and rich.
0:03:34 > 0:03:36The soil of the island is very rich.
0:03:36 > 0:03:41You can see just over here where we grew our own crops and the evidence of the ridges.
0:03:41 > 0:03:43- Those lines on the turf?- Yeah.
0:03:43 > 0:03:47It was fantastic for the potatoes and all the vegetables.
0:03:47 > 0:03:51You had to be able to turn your hand to everything, living on an island.
0:03:53 > 0:03:58The fertile soil is a clue to how the extraordinary landscape of Clew Bay formed.
0:03:58 > 0:04:03Its islands are made of the rich residue left behind by glaciers.
0:04:05 > 0:04:1120,000 years ago, much of Ireland was covered by a vast ice sheet.
0:04:11 > 0:04:16As the climate cooled, and warmed, the ice advanced and retreated, moulding the land underneath,
0:04:16 > 0:04:22and creating the distinctive features that became Clew Bay.
0:04:22 > 0:04:28Paul Dunlop is an expert on how glaciers made the mounds which formed these islands.
0:04:28 > 0:04:30These are known technically as drumlins,
0:04:30 > 0:04:32but where does the word come from?
0:04:32 > 0:04:35The word drumlin comes from the Gaelic word druim,
0:04:35 > 0:04:36which means a small hill.
0:04:36 > 0:04:42Any glacial landscape you go to, you find these landforms. They're always called drumlins.
0:04:44 > 0:04:49What's so striking is the repetitive pattern of drumlin islands across the bay.
0:04:49 > 0:04:52Paul's developed a theory that a wave-like motion
0:04:52 > 0:04:56under the melting ice created these distinctive shapes and patterns.
0:04:56 > 0:05:00It's a process similar to what happens when the tide goes out on a beach,
0:05:00 > 0:05:03leaving those familiar wave-like ripples in the sand.
0:05:03 > 0:05:07If you take a look around nature, you find wave patterns everywhere.
0:05:07 > 0:05:10- You find them in the clouds, on the beach.- Ripples on sand.
0:05:10 > 0:05:15Yes, exactly, and ice flowing across sediment can produce the same scenario.
0:05:15 > 0:05:22As the wave goes up, it's leaving sediment on the surface of the land, which then becomes a drumlin.
0:05:22 > 0:05:25That's right.
0:05:25 > 0:05:29It is amazing that the most brutal forces working deep beneath the ice so long ago
0:05:29 > 0:05:34left us their legacy - this beautiful bay.
0:05:38 > 0:05:44For seafarers who know these islands and reefs, it's a place of protection from the North Atlantic.
0:05:44 > 0:05:48But without local knowledge, it's also a treacherous maze.
0:05:51 > 0:05:58400 years ago, this territory was controlled by an extraordinary Gaelic leader who lived in this.
0:06:02 > 0:06:06The Tower House at Rockfleet sits on a natural slab of bedrock.
0:06:06 > 0:06:12And at high tide, it's surrounded on three sides by water.
0:06:13 > 0:06:15- Hello.- Hello there!
0:06:15 > 0:06:16Can I come in?
0:06:16 > 0:06:20You're more than welcome, but mind your head.
0:06:20 > 0:06:24Denise Murray knows every nook and cranny of the Rockfleet Tower House.
0:06:24 > 0:06:28But first, I have to find her in this warren of a castle.
0:06:30 > 0:06:33Each floor has a spacious room, but the passages and stairways
0:06:33 > 0:06:37twist and turn, as well as being unbelievably narrow.
0:06:37 > 0:06:40Who's the most famous occupant of here, then?
0:06:40 > 0:06:42The main occupant of this tower house
0:06:42 > 0:06:45was a woman named Grainne ni Mhaille who lives on in legend
0:06:45 > 0:06:47as the Pirate Queen of Connaught,
0:06:47 > 0:06:51which does her a disservice, because she was much, much more than that.
0:06:51 > 0:06:57She was a trader, pirate, mother, grandmother and the wife of the man
0:06:57 > 0:07:00who eventually became the overlord of Mayo, with her financial backing.
0:07:00 > 0:07:04- Will we go further up? - Yes, mind your head.
0:07:04 > 0:07:09Very impressive that the most famous occupant here is a woman.
0:07:09 > 0:07:12- Yes.- To be remembered from that time.
0:07:12 > 0:07:16Grainne ni Mhaille, the Pirate Queen,
0:07:16 > 0:07:21is sometimes referred to by an Anglicised version of her name, Grace O'Malley.
0:07:21 > 0:07:28Grace saw the sea as her domain, so anyone who crossed it was fair game.
0:07:28 > 0:07:31She would stand here, having come up from her hall, and look out across Clew Bay.
0:07:31 > 0:07:37And she would see a ship. And down below, she had three galleys, 200 fighting men,
0:07:37 > 0:07:42with oar and sail, and they would take over across the bay like rockets
0:07:42 > 0:07:44and capture whoever was passing.
0:07:44 > 0:07:50She particularly despised the merchants of Galway, who had a monopoly on the wine trade.
0:07:50 > 0:07:56Many a Galway-bound merchant ship fell pray to Grace O'Malley's ships.
0:07:56 > 0:07:59Eventually, they came looking for her.
0:07:59 > 0:08:05She could defend this castle from attack, which she did in 1579,
0:08:05 > 0:08:08where ships were sent from Galway to arrest her because of her piracy.
0:08:08 > 0:08:11And she beat them off, so much so that the man in charge
0:08:11 > 0:08:15of the expedition said he was afraid she was going to capture him.
0:08:15 > 0:08:18This is warriorship. She had the values
0:08:18 > 0:08:22a martial society valued, she just was a woman and a mother.
0:08:24 > 0:08:29Grace brought up her children here, and although the tower would have had home comforts,
0:08:29 > 0:08:34its primary purpose was to protect the O'Malleys from their enemies.
0:08:34 > 0:08:35And what are these for?
0:08:35 > 0:08:39They're quite simply for dropping things down on top of people.
0:08:39 > 0:08:45Grainne's standing here, her castle is under attack, the last thing she wants them to do is get in the door.
0:08:45 > 0:08:48So she's here, they've got oil, they've got pitch,
0:08:48 > 0:08:52they've got anything that will burn or anything that is disgusting.
0:08:52 > 0:08:53And they pour it down here.
0:08:56 > 0:08:59In the O'Malley house, security was paramount.
0:08:59 > 0:09:05Even if attackers got into the ground floor, Grace had installed another line of defence.
0:09:09 > 0:09:13Instead of a stone staircase, there was a wooden ladder that could be removed.
0:09:13 > 0:09:19And even if they got past that, there was another surprise in store for any 16th-century raiders.
0:09:19 > 0:09:23- This is not an easy building to get around, is it? - No, and deliberately so.
0:09:23 > 0:09:27To get through that door, even somebody as short as me has to bend down to come through.
0:09:27 > 0:09:32Obviously, a fully armoured man in here has the advantage, he can just kill you.
0:09:32 > 0:09:36So what they would do is, if you had managed to get up those wooden stairs,
0:09:36 > 0:09:41the first person up would be caught, their throat would be cut and they'd be thrown back,
0:09:41 > 0:09:47it's called the murder hall, onto their comrades below as a little disincentive
0:09:47 > 0:09:50- to come any further. - This is one wild country.
0:10:04 > 0:10:10It's the wildness of the ocean that dominates now as we journey north-west to Achill Island.
0:10:11 > 0:10:16Massive marine ramparts speak of the power struggle between land and sea.
0:10:21 > 0:10:25People, too, have left their mark in stone.
0:10:25 > 0:10:28The remains of communities who finally conceded defeat
0:10:28 > 0:10:31in an age-old battle to cling on to this coast.
0:10:34 > 0:10:39Further around the coast of County Mayo, communities still thrive at Beal Derrig.
0:10:47 > 0:10:50Beal Derrig doesn't have a village centre as such.
0:10:50 > 0:10:54Each family home is surrounded by fields,
0:10:54 > 0:10:56precious land for farming.
0:10:58 > 0:11:01It's an agricultural tradition that goes way, way back.
0:11:07 > 0:11:11Alice is time-travelling back to its beginnings.
0:11:11 > 0:11:15Underneath my feet are the preserved remains
0:11:15 > 0:11:18of the oldest farm site in the British Isles.
0:11:18 > 0:11:25The discovery was made back in 1934 when this man, Patrick Caulfield, was cutting peat
0:11:25 > 0:11:30in these fields and kept on striking stones buried in a regular pattern.
0:11:30 > 0:11:32Patrick's son, archaeologist Seamus Caulfield,
0:11:32 > 0:11:38has continued his father's investigation into the stones beneath the bog.
0:11:38 > 0:11:43Seamus came up with this very simple technique of probing to plot their locations.
0:11:43 > 0:11:45The probe goes through easily, doesn't it?
0:11:45 > 0:11:47So, what am I hitting there, Seamus?
0:11:47 > 0:11:49You're hitting ground level,
0:11:49 > 0:11:52and now we're hitting on something higher.
0:11:52 > 0:11:55- You can actually hear it hitting on the stone.- I can.
0:11:56 > 0:12:00The deeper you probe the peat, the further back in time you go.
0:12:00 > 0:12:05The depth and pattern of the finds forced Seamus and his father to an astounding conclusion.
0:12:05 > 0:12:10The stones were placed here before Stonehenge.
0:12:10 > 0:12:15That's a stone that someone lifted into place 5,500 years ago.
0:12:15 > 0:12:19It hasn't been seen or known about for 5,000 years.
0:12:19 > 0:12:23- And we're hearing it now for the first time. - That is amazing.
0:12:23 > 0:12:29Mapping the site, they realised they might be following the lines of buried walls.
0:12:29 > 0:12:32We're hitting a wall in section, are we? Do you think?
0:12:32 > 0:12:38Yes, we're coming across the wall and it should now begin to drop the far side of it.
0:12:43 > 0:12:47Some of this massive site has been excavated to confirm the theory
0:12:47 > 0:12:51that the lines of stones plotted with all that probing
0:12:51 > 0:12:57were collapsed walls that would originally have stood around a metre high and a metre wide.
0:13:00 > 0:13:06These buried walls once marked out the British Isles' oldest network of farmers' fields.
0:13:09 > 0:13:12They extend over this mountain, over the mountain in the distance,
0:13:12 > 0:13:18and they're large, enclosed fields, appear to be for cattle, grazing land for cattle.
0:13:20 > 0:13:23It's likely that 5,500 years ago
0:13:23 > 0:13:29people were engineering the landscape here to rear animals for food.
0:13:29 > 0:13:34These are the fields of Ireland's first farmers.
0:13:34 > 0:13:39The long parallel walls run all the way from the cliff edge for half a mile inland.
0:13:39 > 0:13:43The layout suggests that cattle were reared here for meat and milk,
0:13:43 > 0:13:48because walled fields meant the farmers could separate stock and control grazing.
0:13:48 > 0:13:53This extensive farm would have supported as many as 1,000 people.
0:13:53 > 0:13:55So this is a massive undertaking.
0:13:55 > 0:14:00People must have been working as a team to build all these miles and miles of stone walls.
0:14:00 > 0:14:04They had to be. It's not a single operation, it's not a few families.
0:14:04 > 0:14:07It's a large community, making a decision
0:14:07 > 0:14:11to divide the terrain like this into these long, large fields.
0:14:11 > 0:14:14Someone was making the decision and they were sticking to it.
0:14:22 > 0:14:24Subtitles by Red Bee Media Ltd