Croagh Patrick to Beal Derrig Coast

I'm on the holy mountain of Croagh Patrick,

where St Patrick is said to have fasted for 40 days.

Once a year, thousands of pilgrims make the climb

to the 762-metre summit.

Many of them in bare feet.

Some Catholics brave the pain of this barefoot pilgrimage as a penance.

But I'm here on a mission of my own.

The pilgrimage I'm making is to celebrate one of nature's great spectacles,

and you need to get high up to take it in.

The extraordinary islands of Clew Bay.

It's a beguiling water world, unlike anything else in the British Isles.

Local mythology counts Clew Bay's islands at 365.

One for every day of the year.

I'm intrigued to discover how this community of islands once supported a community of people.

Mary Gavin-Hughes still sails these waters.

She's one of the last generation of self-sufficient islanders who've fished, and farmed, in Clew Bay.

What was it like living on the Islands?

It was heaven on Earth living on the island.

It was very peaceful, great tranquillity.

Mary grew up in a world with no electricity, in a tight-knit community separated by water.

What's that building over there?

This one here is, erm, known as Cullen school, that's Cullen Island, that was the school.

-That little white building?

-Yep.

That's the smallest school I've ever seen in my life!

By the time Mary was a teenager, she was roving around Clew Bay on her own.

This picture here shows how we used to row to and from home.

Here you are. It's a heavy looking boat, these oars are absolutely huge.

They were handmade, my dad actually made them, and, erm...

Yeah, they were good and sturdy, but we needed them

for the weather we were up against sometimes.

-You look as though you're enjoying yourself there.

-Of course I am.

Smile, Charlie.

That's his home.

Mary's father taught her to feel at home on the water, harvesting the sea's bounty.

But they didn't live on fish alone.

We did all our farming on the island, our fishing, and we were very self-sufficient.

The grass seems really quite lush and rich.

The soil of the island is very rich.

You can see just over here where we grew our own crops and the evidence of the ridges.

-Those lines on the turf?

-Yeah.

It was fantastic for the potatoes and all the vegetables.

You had to be able to turn your hand to everything, living on an island.

The fertile soil is a clue to how the extraordinary landscape of Clew Bay formed.

Its islands are made of the rich residue left behind by glaciers.

20,000 years ago, much of Ireland was covered by a vast ice sheet.

As the climate cooled, and warmed, the ice advanced and retreated, moulding the land underneath,

and creating the distinctive features that became Clew Bay.

Paul Dunlop is an expert on how glaciers made the mounds which formed these islands.

These are known technically as drumlins,

but where does the word come from?

The word drumlin comes from the Gaelic word druim,

which means a small hill.

Any glacial landscape you go to, you find these landforms. They're always called drumlins.

What's so striking is the repetitive pattern of drumlin islands across the bay.

Paul's developed a theory that a wave-like motion

under the melting ice created these distinctive shapes and patterns.

It's a process similar to what happens when the tide goes out on a beach,

leaving those familiar wave-like ripples in the sand.

If you take a look around nature, you find wave patterns everywhere.

-You find them in the clouds, on the beach.

-Ripples on sand.

Yes, exactly, and ice flowing across sediment can produce the same scenario.

As the wave goes up, it's leaving sediment on the surface of the land, which then becomes a drumlin.

That's right.

It is amazing that the most brutal forces working deep beneath the ice so long ago

left us their legacy - this beautiful bay.

For seafarers who know these islands and reefs, it's a place of protection from the North Atlantic.

But without local knowledge, it's also a treacherous maze.

400 years ago, this territory was controlled by an extraordinary Gaelic leader who lived in this.

The Tower House at Rockfleet sits on a natural slab of bedrock.

And at high tide, it's surrounded on three sides by water.

-Hello.

-Hello there!

Can I come in?

You're more than welcome, but mind your head.

Denise Murray knows every nook and cranny of the Rockfleet Tower House.

But first, I have to find her in this warren of a castle.

Each floor has a spacious room, but the passages and stairways

twist and turn, as well as being unbelievably narrow.

Who's the most famous occupant of here, then?

The main occupant of this tower house

was a woman named Grainne ni Mhaille who lives on in legend

as the Pirate Queen of Connaught,

which does her a disservice, because she was much, much more than that.

She was a trader, pirate, mother, grandmother and the wife of the man

who eventually became the overlord of Mayo, with her financial backing.

-Will we go further up?

-Yes, mind your head.

Very impressive that the most famous occupant here is a woman.

-Yes.

-To be remembered from that time.

Grainne ni Mhaille, the Pirate Queen,

is sometimes referred to by an Anglicised version of her name, Grace O'Malley.

Grace saw the sea as her domain, so anyone who crossed it was fair game.

She would stand here, having come up from her hall, and look out across Clew Bay.

And she would see a ship. And down below, she had three galleys, 200 fighting men,

with oar and sail, and they would take over across the bay like rockets

and capture whoever was passing.

She particularly despised the merchants of Galway, who had a monopoly on the wine trade.

Many a Galway-bound merchant ship fell pray to Grace O'Malley's ships.

Eventually, they came looking for her.

She could defend this castle from attack, which she did in 1579,

where ships were sent from Galway to arrest her because of her piracy.

And she beat them off, so much so that the man in charge

of the expedition said he was afraid she was going to capture him.

This is warriorship. She had the values

a martial society valued, she just was a woman and a mother.

Grace brought up her children here, and although the tower would have had home comforts,

its primary purpose was to protect the O'Malleys from their enemies.

And what are these for?

They're quite simply for dropping things down on top of people.

Grainne's standing here, her castle is under attack, the last thing she wants them to do is get in the door.

So she's here, they've got oil, they've got pitch,

they've got anything that will burn or anything that is disgusting.

And they pour it down here.

In the O'Malley house, security was paramount.

Even if attackers got into the ground floor, Grace had installed another line of defence.

Instead of a stone staircase, there was a wooden ladder that could be removed.

And even if they got past that, there was another surprise in store for any 16th-century raiders.

-This is not an easy building to get around, is it?

-No, and deliberately so.

To get through that door, even somebody as short as me has to bend down to come through.

Obviously, a fully armoured man in here has the advantage, he can just kill you.

So what they would do is, if you had managed to get up those wooden stairs,

the first person up would be caught, their throat would be cut and they'd be thrown back,

it's called the murder hall, onto their comrades below as a little disincentive

-to come any further.

-This is one wild country.

It's the wildness of the ocean that dominates now as we journey north-west to Achill Island.

Massive marine ramparts speak of the power struggle between land and sea.

People, too, have left their mark in stone.

The remains of communities who finally conceded defeat

in an age-old battle to cling on to this coast.

Further around the coast of County Mayo, communities still thrive at Beal Derrig.

Beal Derrig doesn't have a village centre as such.

Each family home is surrounded by fields,

precious land for farming.

It's an agricultural tradition that goes way, way back.

Alice is time-travelling back to its beginnings.

Underneath my feet are the preserved remains

of the oldest farm site in the British Isles.

The discovery was made back in 1934 when this man, Patrick Caulfield, was cutting peat

in these fields and kept on striking stones buried in a regular pattern.

Patrick's son, archaeologist Seamus Caulfield,

has continued his father's investigation into the stones beneath the bog.

Seamus came up with this very simple technique of probing to plot their locations.

The probe goes through easily, doesn't it?

So, what am I hitting there, Seamus?

You're hitting ground level,

and now we're hitting on something higher.

-You can actually hear it hitting on the stone.

-I can.

The deeper you probe the peat, the further back in time you go.

The depth and pattern of the finds forced Seamus and his father to an astounding conclusion.

The stones were placed here before Stonehenge.

That's a stone that someone lifted into place 5,500 years ago.

It hasn't been seen or known about for 5,000 years.

-And we're hearing it now for the first time.

-That is amazing.

Mapping the site, they realised they might be following the lines of buried walls.

We're hitting a wall in section, are we? Do you think?

Yes, we're coming across the wall and it should now begin to drop the far side of it.

Some of this massive site has been excavated to confirm the theory

that the lines of stones plotted with all that probing

were collapsed walls that would originally have stood around a metre high and a metre wide.

These buried walls once marked out the British Isles' oldest network of farmers' fields.

They extend over this mountain, over the mountain in the distance,

and they're large, enclosed fields, appear to be for cattle, grazing land for cattle.

It's likely that 5,500 years ago

people were engineering the landscape here to rear animals for food.

These are the fields of Ireland's first farmers.

The long parallel walls run all the way from the cliff edge for half a mile inland.

The layout suggests that cattle were reared here for meat and milk,

because walled fields meant the farmers could separate stock and control grazing.

This extensive farm would have supported as many as 1,000 people.

So this is a massive undertaking.

People must have been working as a team to build all these miles and miles of stone walls.

They had to be. It's not a single operation, it's not a few families.

It's a large community, making a decision

to divide the terrain like this into these long, large fields.

Someone was making the decision and they were sticking to it.

Subtitles by Red Bee Media Ltd