(Mac) DURKAN ... The word duarcáin, from which Durkan is presumably derived, means a pessimist. Durkan is a very numerous name in Mayo and Sligo: it is estimated that there are well over 2,000 persons so called in north Connacht and very few elsewhere. It originated in Co. Sligo and is said to have been adopted by a branch of the O'Haras: a Duarcán who died in 1225 does appear in the O'Hara pedigree. (p. 87, 1984)
The name is also found in Sloinnte Gaedheal is Gall:
Son of Duarcán: the name of a well-known north Connacht family who were anciently lords of Cuil Neiridh, in Co. Sligo, and are probably a branch of the O'Haras. This surname is sometimes pronounced Ó Cuarcáin in the spoken language of Mayo.
One source lists the name Durkin as originating from the Gaelic Dalcassian tribe, and has name originating from the word dobharcu/dobharcon, meaning water hound or otter. ( 'Irish Pedigrees' by O'Hart, Volume 1, Page 86, dated 1892, reprinted 1989.)
So far there is no evidence linking the Durkins to the Dál gCais. Initial DNA information and early documentation points to the Durkin surname originating in or near the barony of Gallen in county Mayo, named after the medieval Irish Gailenga tribe. There is a townland in Gallen named Laghtmacdurkan, which indicates that it is a memorial site to some one with the name "MacDurkan".
To date there have been no matches to anyone with the surname of O'Hara, bringing into question the relationship between the O'Haras and Durkans. There may be a connection, perhaps from a maternal line rather than the paternal line. The established surname clusters that show a link with the existing Durkins are Kivlin/Kivlehan and one branch of the Mulvihills. According to MacLysaght, the Kivlehan (Cibhleachain) name is reported to have originated in county Westmeath, but is now primarily found in county Sligo, where Durkin is also very common. The Mulvihills (MaoilMhichil) were originally from county Roscommon, however in the 15th century, they were evicted and resettled mostly in the counties of Longford, Clare, Kerry and Limerick. The connections of Mac Duarcáin to the Ó Cibhleachain and MaoilMhichil surnames were almost certainly before each surname came into common use (circa 600-1000 years ago). These connections may show a possible migration path from the east midlands of Ireland into Connacht, where most of the Durkin Y-DNA members and close matches have their origins.
Special thanks to Adrian Martyn for providing additional research on the Durkin surname and the history of Mayo and the Gailenga.