Best Field Guide, Ireland (1 Viewer)

I'm planning a trip to Ireland in September, and I'm looking for
opinions on the best guide to carry. I'll be spending a few days
in Wales at the end of the trip, as well as returning to other parts
of the UK in a couple of years, so if there's a guide that covers both
Ireland and the UK, that would be ideal.

The answer has to be Collins. This covers not only Ireland and Britain, but the rest of Europe and North Africa as well.

Collins Bird Guide (Mullarney, Svensson, Zetterstrom, Grant) or Birds of Europe (Jonsson) are the best and most comprehensive. For something a little more pocketable try A Pocket Guide to the Birds of Britain and North-West Europe (Madge, Nurney). A comprehensive Irish guide is The Complete Guide to Ireland's Birds (Dempsey, O'Clery), which is a large format hardback - not good for ID but gives an idea of what is present in Ireland and has distribution maps. It was edited down to form the small paperback A Pocket Guide to the Common Birds of Ireland. For where to see birds, try Where to Watch Birds in Ireland (Hutchinson).