After seeing the bird recently Tristan Reid suspected it could be aTaverners Canada Goose. He kindly forwarded these comments
"Firstly this is a large bird (either the same size or slightly larger than barnacle goose). The body length seemed very long; looking out of proportion with rest of the bird (a feature also noted in the D&G bird - certainly not the chunky/compact structure expected in Richardson's). The bill looked reasonably long (not as short and stubby as I would expect from a Richardson's). Even though the black line separating the white 'chin' (shown in this bird) is not a 'clincher' for Taverner's (as this feature is sometimes seen in other forms) I have yet to see a Taverner's candidate lacking this feature! The Sligo bird looks remarkably similar to the D&G bird (though the Sligo bird may possibly have a slightly paler upper breast) and so I would be fairly happy in calling it a Taverner's-type."
Tristan has kindly forwarded photos of a similar bird seen previously in Dumfries and Galloway. Nice photos!
American birder Steve Mlodinow commented via M.O'Keefe "The relitivaly smooth contour of the head shape, the "swelling" at the base of the beak, the birds relitive bulkiness and the dingy breast colour all fit nicely with Taverners, not Richardsons.Also the great majority of Richardsons have an indentation on their white cheek which the Irish bird lacks"
More pictures of the Sligo Taverners Goose here:
Many thanks to Tristan, Mike and Steve. Also to Chris Baines who first flagged it as a possible.