The Mac Diarmada were the ruling dynasty of Magh Luirg (Moylurg; northeast Connacht) from the 10th to 16th centuries. A castle stood on this island from the 12th century: in 1184, the Annals of Loch Cé report that a lightning bolt caused a fire:

The Rock of Loch-Cé was burned by lightning, i.e. the
very magnificent, kingly residence of the descendents of Máel Ruanaid where neither goods nor people of all that were
there found protection; where six score, or seven score, of
distinguished persons were destroyed, along with fifteen
men of the race of kings and chieftains, with the wife of
Mac Diarmada, i.e. the daughter of Ó hEidhin, and his
son's wife, i.e. the daughter of Domhnall O'Conchobhair,
and the daughter of Ó Dubhda, and the son of Donnchadh
O'Maelbhrenuinn, and the son of Donn O'Mannachain,
and the two daughters of O'Mannachain, and Mac Maenaigh,
chieftain of Cenél-Builg, and the priest O'Maelbealtaine,
and Gillachiarain Ó Connachtain, (i.e. a son of
chastity and lamp of piety), and a countless destruction
besides of good men; and every one of them who was not
burned was drowned in this tumultuous consternation,
in the entrance of the place; so that there escaped not
alive therefrom but Conchobar mac Diarmata with a
very small number of the multitude of his people.[1]

A rebuilt castle featured in the final part of the 1235 conquest of Connacht by Richard Mór de Burgh, 1st Baron of Connaught. The castle came under siege, first by a raft-mounted catapult, and then by fire ships. Cormac MacDermott, King of Moylurg, was forced to surrender.

A poem addressed to Tomaltach an Einigh mac Diarmata (King of Moylurg 1421–58) tells the story of the Hag of Lough Key who used (or abused) Cormac MacDermott's (king 1218–44) hospitality by staying on the Rock for a full year, and laid upon the McDermotts the obligation of perpetual hospitality.

The McDermotts lost the island in 1586, and Eochaidh Ó hÉoghusa (1567–1617) wrote a poem lamenting the castle's emptiness.[2]

By the early 19th century the castle was in ruin, until architect John Nash was commissioned to redevelop the structure as a summer house or folly.[3][4]

In 2014, the island and castle featured in an episode of sitcom Moone Boy, as the residence of the mysterious "Island Joe."

In 2018, the castle was put up for sale for €80,000.[5] However, the sale was later withdrawn and the castle returned to private Irish ownership.[6]

In 2019, archaeological excavations were also undertaken on the island.[7] The excavations concluded that the island itself is essentially a multi-period fortification, with the earliest elements dating to the early medieval era. An earlier, two to three meter thick enclosure wall pre-dating the standing enclosure wall was excavated at a depth of a meter and a half. Medieval buildings dating to the thirteenth century were excavated on the northern side of the island above the earlier fortification, but were not found to be connected to the standing enclosure wall. High-status Gaelic artifacts were discovered, including silver pins, a gaming piece, and large collections of butchered cattle, boar, and sheep. The research on the island was featured in Archaeology Magazine.[8]