Mac Gilla Mo Cholmóc, Diarmait (d. 1216), king of Uí Briúin Chualann, was the son of Domnall Mac Gilla Mo Cholmóc (qv) (d. c.1185) and his wife, Derbforgaill, and succeeded to the kingship on his father's death. Named after his maternal grandfather, Diarmait Mac Murchada (qv) (d. 1171), he continued his father's policy of cooperation with the English; during his lifetime the pace of feudalization and anglicization of the Mac Gilla Mo Cholmóc dynasty gathered momentum.
In the early years after the English arrival, Domnall's charters had featured Ostmen, Leinster aristocrats, and Englishmen, but by the late twelfth century a shift had occurred and Diarmait was granting lands to newcomers such as Richard de Felde in charters dominated by English witnesses. Diarmait described himself in documents as ‘Dermod filius Gillemolmoc’ and received lands as ‘Dermot Mac Gilmeholmoc’ in 1207. The benevolence of the English to the Mac Gilla Mo Cholmóc dynasty greatly facilitated the feudal settlement in the area south of Dublin, where the colonists prospered, living alongside the traditional lordship. In Uí Briúin Chualann itself his descendants the FitzDermots held at least eight carucates, while in 1207 Diarmait is recorded as holding another fifteen carucates in the Vale of Dublin in return for one knight's fee and two otter skins.
Until 1215, at least, the lands of the Meic Gillamocholmóc stretched to Glencullen and included Newcastle Lyons; grants made by Diarmait of lands to Glendalough and St Mary's Abbey, Dublin, further display the extent of his holdings. South of Uí Briúin Chualann, Diarmait and his dynasty seemingly also gained additional lands at the expense of both Ostman and Irish dynasties, particularly the MacTurkills dwelling within Uí Briúin Chualann and the former O'Farrell rulers of Uí Garrchon (east Wicklow).
Diarmait died about 1216 and was succeeded as king of Uí Briúin Chualann by his son John FitzDermot (Mac Gilla Mo Cholmóc) (d. 1230), whose customary name form bears witness to the alignment of this Irish dynasty with the English hegemony. Further evidence of their assimilation is provided by Henry III's summons in 1227 of John and other magnates of Ireland to perform military service. John also eased his passage into the colonial aristocracy through his marriage to Claricia, daughter of Gilbert FitzGriffin of Knocktopher, Co. Kilkenny; they gave their children English forenames. The transformation was so complete that, without the evidence of their Irish lineage, the FitzDermots would have been indistinguishable from the settler aristocracy. In Uí Garrchon John and his successors held further lands at Newcastle and at Ballinagran (probably at the expense of the O'Farrell dynasty) into the fourteenth century. John FitzDermot died about 1230 and was succeeded by his son John fitz John FitzDermot (c.1205–64).