McDermott's Castle
Caisleán Mhic Diarmada
Ireland · County Roscommon · Near Boyle
Built 1180 · Island castle ruin — Norman origin, 12th century foundation on a natural lake island; rebuilt and extended by the MacDermott-Roe dynasty through the 14th–17th centuries; the surviving structures include curtain wall fragments and a square tower; the island setting is the defining characteristic
This page is part of an independent travel guide and is not affiliated with, endorsed by, or operated by McDermott's Castle.
© Castles & Palaces
Quick Facts
- Hours
- Closed Mon–Sun
- Duration
- 1.5–2 hours (boat tour from mainland)
- Best time
- May to September
- Nearest city
- Boyle
Highlights
- ✦Castle Island setting — McDermott's Castle occupies its own island on Lough Key, accessible only by boat, giving it one of the most romantic silhouettes in Connacht
- ✦Guided boat tour of Lough Key — the castle is the focal point of a scenic cruise that covers the lake's 33 islands and their history
- ✦MacDermott dynasty history — lords of Moylurg (north Roscommon) for 500 years, patrons of the Annals of Lough Cé (one of the most important medieval Irish annals), and the last Gaelic family in Roscommon to maintain a hereditary castle
- ✦Lough Key Forest Park — the former estate surrounding the lake, now a woodland activity park with a bog garden, tree canopy walk, and archaeological sites on the mainland shore
- ✦Romantic tragedy of McDermott and Mary Cormac — a 19th-century island love story that became one of the most famous in Irish tradition
Skip the queue with a guided tour
Skip-the-line tickets & expert guides
McDermott's Castle stands on Castle Island in Lough Key — a 33-island lake in north Roscommon surrounded by one of Ireland's most beautiful stretches of Atlantic woodland. The castle is a ruin: curtain wall fragments and a square tower survive on the island, together with the remains of a small church. What makes the site exceptional is its setting — the castle on its private island, reflected in the dark water of Lough Key, with the forested hills of Boyle surrounding the lake, constitutes one of the most atmospheric medieval landscapes in the west of Ireland.
The island is not accessible to independent visitors — there is no public landing stage, and the castle ruins are not maintained as a visitor attraction in the conventional sense. The closest legitimate access is the Lough Key guided boat tour (GetYourGuide, t1374909, €22), which offers lake-level views of the castle and island with commentary from a local guide. The tour is framed as a scenic cruise around the lake with views of McDermott's Castle — not as an admission ticket or landing visit. Visitors looking for views from land can access the Lough Key Forest Park on the mainland shore, where the castle is visible across the water from several points on the shoreline path.
History
The MacDermott-Roe family — Mhic Diarmada in Irish, lords of Moylurg (roughly north Roscommon and east Mayo) — established their castle on the largest island of Lough Key sometime in the 12th century, possibly on the site of an earlier Norman fortification. The lake's position — surrounded by woodland and approachable only by boat — gave the island castle an inherent defensive advantage that the MacDermotts exploited for five centuries.
The family were major patrons of Irish learning: the Annals of Lough Cé, one of the most important chronicles of medieval Irish history covering the period 1014–1590, was compiled at Lough Key under MacDermott patronage. The castle remained the family seat through the Tudor conquest and its aftermath, making the MacDermott-Roe lords among the last Gaelic dynasties in Connacht to maintain a hereditary castle. They were finally displaced during the Cromwellian land settlements of the 1650s.
The most poignant chapter of the castle's history is 19th-century: Una MacDermott and Thomas Costello (also told as Mary Cormac and MacDermott) — a love story between a Protestant Irish landlord and a Catholic tenant's daughter, separated by family opposition, that ended in both their deaths and became one of the most celebrated romantic tragedies in Irish oral tradition. Thomas Costello allegedly swam out to the island nightly to see Una; after her death, her father refused to bury her on the island, and Costello died of grief on the mainland shore. Both are said to be buried on the island. The story inspired the poem 'Dark Rosaleen' and is still told by local guides.
How to Visit
Getting there: Lough Key Forest Park is 3 km east of Boyle town on the N61. Boyle is accessible by train from Dublin (via Mullingar, approximately 2 hours) or by bus from Galway and Sligo. A car is the most practical option for reaching the lake from the nearest train station.
Tickets: Castle Island has no public landing facility — independent entry is not possible. The Lough Key guided boat tour (t1374909, from €22) departs from the Lough Key Forest Park marina and offers lake-level views of the castle with local historical commentary; run April through October. Lough Key Forest Park (separate entrance fee) provides the mainland shoreline viewpoints.
Practical tips: The GYG boat tour is a scenic cruise around the lake's 33 islands with views of McDermott's Castle from the water — it is not an admission ticket for castle entry; read the listing carefully before booking. The Lough Key Forest Park is worth the visit on its own — the woodland trails, canopy walk, and bog garden make it a full half-day, with the castle visible across the water from several shoreline points.
Combine with: [Kylemore Abbey](/castles/ireland/kylemore-abbey) in Connemara, about 1.5 hours west, and [Rock of Cashel](/castles/ireland/rock-of-cashel) in Tipperary extend a western Ireland loop through three very different medieval castle settings — island fortress, Victorian lakeside abbey, and royal rock fortress.
Frequently Asked Questions
No — Castle Island has no public landing facility and the castle ruins are not accessible to independent visitors. The island is viewable from the lake on the Lough Key guided boat tour (GetYourGuide, €22) or from the mainland shoreline of Lough Key Forest Park. Do not attempt to access the island by private boat without explicit permission.
Location
Castle Island, Lough Key, Co. Roscommon, Ireland
Tours & Tickets
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