Is roaming free in Ireland with my UK (Post-Brexit) or US SIM card?

It depends.

• If you are from the UK (Post-Brexit): Most UK carriers (like O2, Vodafone, Three, EE) have reintroduced daily roaming fees (e.g., £2-£6 per day) for Ireland.
• If you are from the USA/Canada/Asia: Your home carrier will charge expensive international rates (e.g., $10-$14 per day).

A Cellesim eSIM is a one-time prepaid fee that is significantly cheaper than these roaming charges.