Irish is compulsory in primary & secondary school.
Irish children go into secondary school at the age of 12/13 rather then 11/12 like the UK, they go into secondary in Year 8 rather then Year 7.
Where England has the GCSEs & A Levels, Ireland has the Junior Cert & the Leaving Cert. The Junior Cert is not exactly the equivalent of the GCSEs and even though children can leave school at the age of 16, they don't have the same options available to them as children in England do, most kids remain in school to do their Leaving Cert. There aren't really any further education colleges, or the equivalent of the sixth form colleges, or alternatives to the Leaving Cert.
For the Leaving Cert, most kids do 7 subjects, English, Maths & Irish are compulsory and you can choose the other 4 subjects but you don't get the same range as you do in the UK.
In Ireland, when they say they're going to college, they mean they're going to University. There are colleges that do Post Leaving Certificate courses but they're not really described as college.