I have always favoured the Scottish theory, mainly because the clan 'paper trail' does not come up with the variant McIlhargy, a fact supported by differing DNA results for McIlhagga and McHarg, McHargue, a shortened form of McIlhargy. Following my recent visit to Northern Ireland I have been in correspondence with the historian and Gaelic scholar in Belfast I mentioned in my last blog, who has kindly said she thinks that even from language that the Scottish view is entirely correct on the derivation of McIlhagga and that confusion with McIlharg(y) would be just that, confusion. Her view is that the most quoted Irish author, MacLysacht was terrific on documentary evidence, but less so on etymology. She quotes the more Gaelic-based Irish surname scholar, the Rev. Patrick Woulfe ('Irish Names and Surnames', 1923) who, though he does not deal with Scotland, gives the Irish surname formation Mac Giolla [Mo] Chuda, anglicised as McGillyCuddy (as in McGillycuddy's Reeks) but also in the shortened form as McElcuddy and McElhuddy. She adds 'If you accept that -d- could be changed to -g- (a slight surprise)', but we may suppose the effect of migrating across the North Channel, 'McElhuddy is a good Irish comparison with the Scottish McIlhagga'.
My friend from Belfast also adds that she doesn't know why the personal name Giolla [Mo] Chuda became Gilmagu in Scotland, which is part of the theory propounded by the main 'Scottish' author, G.F. Black, though she admits that 'as a set these Gille / Giolla compound names do get abbreviated in surprising ways'. I will write further about Gilmagu in my next blog.