There's been a couple of previous threads on this subject. If you're in a location where EE service is not available, but a foreign network is, and you have roaming enabled - then your phone will connect to that foreign network by design.
This is one of the reasons for the "Welcome to country xyz" messages that are sent when you connect to foreign networks - in parts of north & west Wales, picking up Irish networks in an EE-blackspot, is not unusual.
In the specific case of Ireland, there are no additional roaming charges - https://ee.co.uk/help/mobile/roaming/roaming-costs/countries/ireland - beyond which would be incurred for using your phone on EE's UK network. When you return to EE coverage, your phone should stop roaming fairly soon after.
