St. Fergus of Downpatrick is about as obscure a saint as they come; perhaps his relative anonymity has to do with the fact that there are nine other saints of the same name listed in the martyrology of Donegal alone, let alone other non-martyred Irish-Scotch saints of the same name.
Fergus was descended the royal house of Coelbad, a pre-Christian King of Erin, and became a monk and then missionary abbot, constructing a monastery at Killmbain in County Down. He was later consecrated bishop and ruled the cathedral Church of Druimleithglais (Down), most likely as its first bishop. Fergus died in 583 and his feast is celebrated on March 30th.
The See of Down was later occupied by the famous St. Malachy.