[4] Leabhar Mór Leacáin is written in Middle Irish and was created by Ádhamh Ó Cuirnín, Murchadh Ó Cuindlis, and an anonymous third scribe[3][5] for Giolla Íosa Mór Mac Fhirbhisigh.

After it was seized from Trinity College Dublin by troops under the command of Sir John Fitzgerald, 2nd Baronet, in 1689 during the Williamite War in Ireland, James II of England deposited it at the Irish College, Paris.

[6] In 1787, the Chevalier O'Reilly[clarification needed] returned it to Ireland, where it was at one stage in the possession of Charles Vallancey, who passed it on to the Royal Irish Academy.

These contained a large section devoted to the pedigrees and history of the Norse and Norse–Gaelic families of Ireland, which are nowhere else preserved.

[8] The pages are covered in a greasy substance which makes them transparent and reduces their legibility.