When you arrive in the UK you will get a text from GoMo telling you about the roaming details.
If you look at https://gomo.ie/pricing/, on the bottom is a link to the country list in pdf form. Once you download it you can see that the UK is treated as EEA for some reason which means it's EU roaming allowance.
There EEA country list is very funny, multiple countries in there which are not EEA but hey, maybe they know more than we do!