Dictionary of National Biography, 1885-1900/O'Dempsey, Dermot
O'DEMPSEY, DERMOT (d. 1193), Irish chief, called in Irish writings Diarmait Ua Diomusaigh, was son of Cubroghda O'Dempsey, who died in 1162. He claimed descent from Ros Failghe, eldest son of Cathaoir Mór, king of Ireland in the second century, and was thus of common descent with O'Conchobhair Failghe, from whom Offaly takes its name. He became chief of Clan Mailughra on his father's death. This was the territory of the O'Dempseys, and lay on both banks of the Barrow in the King's and Queen's Counties, and as far as the edge of the great heath of Maryborough. He afterwards became chief of the whole territory of the group of clans allied to his, all descended from Ros Failghe; this territory included not only the modern baronies of East and West Offaly, co. Kildare, but also the baronies of Portnehinch and Tinehinch, Queen's County, and that part of the King's County which lies in the diocese of Kildare and Leighlin. His chief stronghold was a stone fort, afterwards replaced by a castle, of which the ruins remain on the Rock of Dunamase, a hill in the Queen's County which commands a wide view over the lands of his septs. He was the only O'Dempsey who became king of the whole territory, though after his time, owing to the dispossession of O'Connor Faly by the Fitzgeralds, the O'Dempseys were long the chief clan of the district, in which many of them still remain, though they have prospered little since their share in the massacre of Mullachmaisten or Mullaghmast in 1577. Dermot founded in 1178 a Cistercian abbey at Rosglas, co. Kildare, now known as Monastereven, from a more ancient church of St. Eimhín, which stood on the site of the monastery. The abbot sat in the Irish parliament. The site is now occupied by the house of the late Marquis of Drogheda. O'Dempsey died in 1193. He left a son Maelseachlainn, who was killed by O'Maelmhuaidh of Fircal in 1216.
[Annala Rioghachta Eireann, ed. O'Donovan, vol. iii. Dublin, 1851; Leabhar na Gceart, ed. O'Donovan, Dublin, 1847; Cath Muighi Rath, ed. O'Donovan, Dublin, 1842; local Knowledge.]