Revisiting this from another angle, concerning the original Arthur or Atty Devlin of Glenlark.
Patrick Daly says elsewhere that he has "no information about his [Atty's] brothers and sisters" (http://genforum.genealogy.com/devlin/messages/977.html) and that Atty was a native of Garvagh (http://genforum.genealogy.com/devlin/messages/835.html).
This leads me to suggest - speculatively as yet - that Atty may have been one of the family of Peter Devlin, who was displaced from around Pomeroy to Garvagh, where he is found in the Tithe Applotments (1828). Peter is thought to have had seven children: Atty, Michael, Owen, Sam, Peter and Roseann.
Peter (junior) had a son James (died 1949) who was a schoolmaster, and whose own family includes two eminent scholars, Rev Prof Brendan Devlin, retired Professor of French at Maynooth, and Rev Ciarán Devlin, Derry diocesan historian.I am taking my information here from an account in Gaelic of the family written by Fr Ciarán.
I don't know how significant it is that Atty heads the above list of Peter's children, but Fr Ciarán seems to think he was the eldest.Before going on to say that Peter divided his land at Garvagh between Sam and Michael, and that Sam made room for Peter (junior), he notes that Atty had found a place of his own.Possibly in Glenlark?The dates are about right, and there is also a brother Owen as required.
But promising though it may be, this it is still only speculative.