On a quiet stretch of Lough Key in County Roscommon, a ruined castle sits on a small island barely bigger than a tennis court. It’s easy to sail past without a second thought. But look closer, and you’re looking at the site of one of Ireland’s oldest love stories — a tale of devotion so fierce that a man swam across dark lake water, night after night, for years.
The MacDermotts of Lough Key
Lough Key has been inhabited since prehistoric times, but the castle on Castle Island was built by the MacDermott clan — the Lords of Moylurg, and one of the most powerful Gaelic families in Connacht.
The island was the perfect stronghold. Surrounded by water on every side, it was nearly impossible to take by force. The MacDermotts used it as their seat of power — a fortified residence from which they held court over the kingdom of Moylurg for centuries.
The Annals of Lough Cé — a detailed chronicle of Irish history — were written here by monks working under MacDermott patronage. The clan had their own burial ground on nearby Trinity Island, where their dead were laid to rest. This corner of Roscommon was, for a long time, the centre of its own small world.
Una and Tomás — the night swimmer’s story
The legend of Lough Key is not about warfare or conquest. It’s about a woman named Una MacDermott and a man named Tomás Láidir Costello.
Tomás was strong — his nickname, Láidir, means “strong” in Irish. But he was not considered a worthy match for Una. Her father, the clan chief, refused him. The exact reason varies with the telling: some say Costello was too poor, others that the clan rivalry ran too deep.
Una died on the island. She was young. Some versions say she died of a broken heart after her father kept them apart; others say illness took her. She was buried on Trinity Island, the family’s burial ground, just across a narrow stretch of water.
What happened next is the part the lake remembers.
Tomás rowed to the island and asked permission to visit Una’s grave. Her father refused. So Tomás swam. Every night, he crossed the water in the dark to pray at her graveside — in rain and cold and silence, month after month, until his body finally gave out.
He died on the island. He was buried beside Una. Local legend says a white flower blooms on her grave each spring, and a red flower blooms on his.
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The castle ruins today
The ruins of McDermott’s Castle still stand on Castle Island, rising from the water in that particular way old Irish stone has — not dramatic, not showy, just quietly there. Ireland has no shortage of extraordinary castles, but few sit in such peaceful surroundings. Thick walls remain intact in places; in others, they have collapsed gently into the lake.
Lough Key Forest Park wraps around the lake on all sides. Run by Coillte, Ireland’s state forestry service, it’s one of the finest forest parks in the midlands — with walking trails through ancient woodland, bog gardens, lake viewpoints, and a treetop walkway. In summer, boat trips let you circle Castle Island and see the ruins up close.
The island is accessible by boat when the park’s rental service is running. The castle ruins are not set up for interior tours, but they’re all the more atmospheric for it — a tumbledown silhouette against open sky and still water.
What else is around Lough Key
You could spend a full day at the forest park and not run out of things to do. But the area around Lough Key rewards wider exploration.
Boyle Abbey is just minutes away in the town of Boyle. Founded in 1161, it’s one of the best-preserved Cistercian ruins in Ireland — remarkably intact and free to visit. The carved Romanesque arches on the nave are worth a long look.
Strokestown Park House is about 25 minutes south. The estate holds Ireland’s National Famine Museum — sobering and essential in equal measure. The walled garden is also one of the finest in the country.
For those who want to experience a real Irish castle overnight, Ashford Castle in County Mayo is about 90 minutes away — one of the most celebrated castle hotels on the island.
Planning your visit
Lough Key Forest Park sits just outside Boyle on the N4 between Dublin and Sligo — about two hours from Dublin and 30 minutes from Sligo town. Entry fees apply and vary by season. Boat trips and water bikes run during summer, so check ahead if water access to Castle Island is the priority.
The town of Boyle is quiet, understated, and genuinely local — with pubs, a handful of restaurants, and a pace that suits the history here. If you’re building a wider Irish castle itinerary, pair Lough Key with Dunluce Castle on the Antrim coast — the dramatic cliff ruin that carries its own dark legend.
Can you visit McDermott’s Castle on Castle Island?
The ruins can be viewed from the water and, during the summer season, reached by boat from Lough Key Forest Park. The castle is a ruin and not set up for interior tours, but boat rental and lake trips are available from the park’s visitor facilities when the season is open.
Is the legend of Una MacDermott and Tomás Costello based on a real story?
The MacDermott family were real — they were the Lords of Moylurg and one of the great Gaelic dynasties of Connacht. The names of Una and Tomás Costello appear in local historical tradition, and the story has been told here for centuries. Whether every detail is literal fact or poetic embellishment is harder to say, but the gravesites on Trinity Island are considered genuine by local tradition.
Is Lough Key Forest Park worth visiting for non-history lovers?
Absolutely. The park is one of the finest in the Irish midlands — with woodland walks, bog gardens, lake views, a treetop walkway, and children’s adventure zones. The castle ruins add depth to the visit, but the park stands on its own as a beautiful day out in a quietly spectacular part of Ireland.
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Ireland is full of ruined castles. Most of them hold only stones. Lough Key holds something harder to explain — the kind of grief that becomes a story, a story that becomes a legend, and a legend that keeps pulling people back, long after the man who swam there is gone.