Expert opinions received so far are on the side of it being a hybrid, possibly Glaucous-winged x Herring Gull.
Nobody in Sligo, including the finder, Derek Charles, put the bird out as anything other than a possible Glaucous-winged Gull.
The note on Irishbirding.com summarises current opinion fairly well "1st Winter type bird showing characteristics of a Glaucous-winged Gull. Drumcliff Bay. Hybrid not ruled out."
I posted a link to some images of the bird last night on the IBN-L, which doesn't seem to have gone through to the list.
Trying again here - images are posted at http://www.sligobirding.com/November2009Gallery.html
It is certainly a very striking looking bird, quite unlike any other gull we have seen in Sligo. The spectacular stranding of tonnes of starfish at Lissadell has attracted about 1,000 gulls showing up in a concentrated area, which yesterday included 1 Kumliens Gull, a first winter Ring-billed Gull, two Iceland Gulls and this bird.
PS Just spotted that I accidentally sent last night's email from my new email address which is not registered for IBN-L mail - my apologies
Begin forwarded message:
> From: Mícheál Casey <[log in to unmask]>
> Date: 14 November 2009 22:29:35 GMT
> To: Irish Bird Network <[log in to unmask]>
> Subject: Re: POSSIBLE Glaucous-winged Gull Sligo
> Hi John, and all,
> Photos of the bird in question are now up on the November Gallery page of the BirdWatch Sligo website.
> See http://www.sligobirding.com/November2009Gallery.html
> Comments welcome.
> Go on, we can take it!
On 15 Nov 2009, at 10:45, Edward Carty wrote:
> Can anyone shed light on this Gull at Lissadell Sligo,Birdguides again had
> something to share this morning and after the !(notable) for our First Red
> flanked Bluetail this week instead of the !!!(mega) they had X(correction) and
> just now an unhappy face(negative news) for the Gull........Has the bird been
> confirmed as a Glaucous winged Gull.......?
> Not that I will be travelling of course...........!