FitzDermot (Mac Gilla Mo Cholmóc), Sir Ralph (c.1225–1291), baron of Rathdown, landowner, son of John fitz John FitzDermot (c.1205–64), and direct descendant of the Mac Gilla Mo Cholmóc, lords of Uí Briúin Chualann, was a prominent figure in the marches of Dublin. The only substantive mention of John fitz John relates to the lands he held at Fynnore and Kellistown in Carlow, which he inherited through his mother Clarissa FitzGriffin, daughter of Gilbert FitzGriffin of Knocktopher in Kilkenny, and a third of which he enfeoffed to William de Dene before 1260. Ralph FitzDermot's career heralded the decline of his family and the contraction of their lands, for by the last decades of the thirteenth century their influence over the Irish of Wicklow had all but disappeared. By 1276–7 Ralph was receiving a fee for his defence of the Vale of Dublin from the Leinster Irish, signifying that the transformation of the Meic Gilla Mo Cholmóc into marcher lords was complete. It was crowned by Edward I's knighting him in 1282, and in the same year he served on a jury that determined the extent of Henry Marshal's lands in Newcastle Lyons. In 1286–7, Sir Ralph pledged his continued allegiance to the English by giving William MacMurrough (Mac Murchada) as hostage for the good behaviour of his followers. By 1291 Sir Ralph was dead; his widow, Joan, married Albert de Kenley, in 1292.
During the minority of Sir Ralph's son, John fitz Ralph FitzDermot (c.1280–c.1330), baron of Rathdown, the FitzDermot estates were given into Kenley's custody. The vulnerability of the FitzDermots was evident in Lent 1301, when the Irish burned east Wicklow, sacking Wicklow, Arklow, and the FitzDermot castle at Rathdown. Some time after 1305 John fitz Ralph sold off eight carucates of land in Uí Briúin Chualann to Nigel le Brun (d. 1309), and in 1306–7 he granted away a carucate at Kilnamanagh near Tallaght. This ebbing away of the FitzDermots’ social capital is also suggested by the naming in 1311 of a Robert fitz Ralph (perhaps John fitz Ralph's brother) and a Robert fitz John as smallholders in the Butler manor of Bray. But they remained thoroughly respectable. John fitz Ralph entered the service of the archbishop of Dublin and served in 1326 as the bailiff of the manor of Shankill, where he probably continued in service until his death around 1330.
Members of the family continued to maintain their access to the common law and their social status within the English lordship, and some FitzDermots managed to preserve themselves as landowners. An unpublished pipe roll of Henry VI records a ‘John son of Dermod’, behind in his rent for Rathdown in 1408, demonstrating the family's remarkable capacity for survival and adaptation.