I doubt you'll find one with unlimited minutes or data. I wouldn't make that a constraint. You can always call people with Skype - not free to call phone numbers, but you can add credit to your Skype account in $10 blocks and then call any European number for a few cents a minute. (Calling cell phones generally costs more.)
But many people in Europe use WhatsApp which is completely free (would use mobile data if not on WiFi). I would set that up for sure if you think you'll be calling people a lot in Europe. If you want to call people at home for free, install Google Voice before you leave. You can call US numbers (even landlines) for free with Google Voice on WiFi without even having a SIM card in the phone. If you get a SIM with data, you can use Google Voice for calls that way too but of course it will use data. You can make phone calls with Google Voice to European numbers just like you can with Skype - by adding some credit to it and then calling for a few cents a minute.
If you get a European SIM, incoming calls are usually free no matter how many minutes you have.
Here's some info on Irish SIM cards. All should work in both Ireland and the UK with EU roaming fees largely done away with:
I have a Dutch Vodafone SIM (never used in the Netherlands, bought on eBay) I've used in Europe for a few trips now. Would work well for you except for limited calling minutes. You can get 10GB of data good for 30 days for 29 euros.