Historically, college meant any group of people who lived under agreed rules, which included educational institutions (though note also the use in "college of cardinals", "college of arm", "electoral college" and some other non-educational uses).
In general current use in Ireland, college would be taken to refer to third-level education. Up until recently, all Irish Universities where split into several different colleges, though Trinity has only one college (it remains that the University of Dublin is legally distinct from its sole constituent Trinity College of Dublin due to it having been founded as the first college of the university after the model of Oxford and Cambridge with the idea of further colleges being founded probably the original goal, though never realised).
The National University of Ireland is similarly composed of different colleges, (University College Dublin, University College Cork, NUI Galway & NUI Maynooth) along with "recognised colleges" such as the Royal College of Surgeons in Ireland, National College of Art and Design and others.
Some other colleges are, for historical reasons I won't go into, now one and the same with their university.
In general, we would talk of "going to college" rather than "going to university".
Now, that said there are secondary schools that have college as part of their name. These fall into four categories:
- Some of the older private (or originally private) secondary schools.
- Schools founded as "technical colleges" with a stronger vocational than academic focus, now mostly called "community colleges".
- Schools focused on the last years of secondary education only, often for people who have decided to repeat their leaving certificate or who have other reasons for being outside of the main state-funded secondary education system.
- Gaelcholáistí, that is secondary schools that teach in Irish. Because coláiste is cognate with the English word college, if one were to Anglicise the name of a Gaelcholáiste one might more likely use college than school. (Comparatively, Gaelscoileanna teach at a primary level so one would be more likely to Anglicise a Gaelscoil's name as school). Generally though, one would just use their Irish name when talking about them in English.
Still, while such schools have college in their name, they'd be normlly referred to as schools, not as colleges. Someone might refer to third case as a secondary-level college due to the older age attending, but they are not what one would think of if one just said "college".