I just got back from a 2-week trip (featuring as much birding as I could shoe-horn in without alienating my wife). We stuck to Kerry and Galway counties primarily (day trips from Killarney, Dingle, & Clifden). In addition to “Finding Birds in Ireland”, which twitchterf recommended, I used eBird, Birdguides.com, and birdingplaces.eu.
eBird, was pretty underutilized, I felt, with a lot of sites having only one or two checklists in the last month (or longer). The latter two websites were intermittently helpful, but definitely brought me to some good spots.
That said, as an American, I found myself to be pretty pleased with whatever I came across. I found that I was adding new birds to my list even when I was “not birding” 😅
I’d also second the sea watching suggestion above. We had a fantastic trip to the area around the Blasket Islands with Blasket Island Marine Ecotours— very knowledgeable naturalist-guide, not just a guy with a few well-worn IDs. Ostensibly I was there to see the puffins, but the sight of Manx Shearwaters exploding out of the water at full speed is what really lodged in my brain. Inishbofin & Inishmaan were also great (though we were both coming down with norovirus on Inishbofin, so my attention was… elsewhere🤢).
I’d be happy to go in to more detail about my experience if you want to message me… don’t want to bore folks with a play by play.