I've come across a George Crossan in the townland of Knockybrin Townland, Aghanunshan.
Considering the dearth of other information, there is quite a lot online about the George Crossan in Knockybrin Townland. He was born possibly about 1781. On the night of 21 October 1859, at the estimated age of 68, he was arrested in Letterkenny by Constable Joseph Keegan for being drunk and was fined 1s5d with 1s costs. He died on 25 March 1867. He's mentioned in Griffiths valuation. My father's name was George Crossan and he came from a long line of George Crossans who originated in Donegal. I'm hoping this Aghanunshan George might be one of ours. Have you had a DNA test?