The life of King Aedh Dubh (Hugh the Black) of Leinster is to be found both in the Annals of the Four Masters and the Annals of Ulster. His name, under the latinized form of Aidus, is to be found in several martyrologies.
His hair colour, rather than any misdeeds, is the source of his designation 'the black'. This distinguishes him from King Aedh Finn of Ossory, Hugh the Fair, on account of his hair colouring - although his deeds were high and holy too.
The great ecclesiastical historian Colgan recounts King Aedh's abdication about the year 591, whereupon he entered the monastery of Kildare for the remaining forty-eight years of his life.
He went on to become Abbot of Kildare and, from 630 to his death in 638 or 639, he was Bishop of Kildare. c.f. Colgan's Trias Thaumaturga, and the Secunda Vita S. Brigidae, cap. xxxv, ps. 523-4. This is a point of singular interest. Some writers ascribe to St. Conleth, and to the Bishops of Kildare after him, a joint role as Bishop-Abbot. However, St. Aedh is the first of the Bishops of Kildare who is recorded as having held both posts.
O'Donovan, in his Annals of the Four Masters vol. i, pps 256-7 gives the year of St. Aedh's death as 638. Colgan gives his feast day as 4th January and prefers the latter year for his birth to Heaven.
St. Aedh of Kildare, pray for us!