If your iPhone 7 is 'unlocked' (I assume it is, but double check), swapping sims is probably the way to go. At least in the UK (and I'm sure Ireland as well), pay as you go sims are pretty easy to come by and fairly inexpensive. When I visit the UK, I'll generally spend about $20 on a sim from a mobile carrier shop (EE or Three are what you'd want in the UK) and that gives me a local number and all the data I can possibly use for a month. (Contrast that with Verizon, which charges me $10/day to use my plan in the UK) It absolutely is plug and play, although you might want to wait around in the shop while it sets itself up in case you need help. Simply swap your Cricket sim back in when you land here at home.
You can buy sim cards online before you go, in corner stores, at the airport, etc. I just find it easier personally to go to a carrier store (think a Verizon or At&T store here) and conduct the transaction there. Better networks, and employees that will help you in case you need it.
Depending on the timing of your trips, you may be able to buy a 1 month sim and use that for both trips- just make sure you don't lose it in between trips. Don't quote me on this, but I'm pretty sure the EU just recently instituted roaming regulations as well- I think you may be able to use a UK sim in Ireland now w/o roaming, and vice versa.