school years and ages

Your child must start full-time education once they reach compulsory school age. This is on 31 December, 31 March or 31 August following their fifth birthday – whichever comes first. If your child’s fifth birthday is on one of those dates then they reach compulsory school age on that date.
For example, if your child reaches compulsory school age on 31 March, they must start full-time education at the beginning of the next term (summer term that year).

Republic of Ireland Class Groups
Year 7 (Secondary)

References:

http://www.rathdownschool.ie/Class-Ages/
http://www.gov.uk/what-different-qualification-levels-mean