I would have assumed it was English in most places to be honest just based on the fluency in countries I've been to. I'm not sure why English isn't considered a foreign language in Ireland also. It might be an official language but it is still foreign.
As far as I know the top two options for languages in secondary schools in Ireland is French and German and I'd consider English a foreign language too.
In the Republic of Ireland, under the Constitution of Ireland, both languages have official status, with Irish being the national and first official language.
In Northern Ireland, English is the primary language for 95% of the population, and de facto official language, while Irish is recognised as an official language and Ulster Scots is recognised as a minority language under the Identity and Language (Northern Ireland) Act 2022.
Yes, in Ireland, all students must study English, Irish and one other foreign language. French is most popular. German used to be second, but it seems that Spanish has now overtaken German.
In Ireland, few people speak Irish. So really, for comparison, French would be the second language studied after Irish, but officially Irish is the native tongue so not counted.