I found this on the ELSPA website. It’s for the UK but mentions Europe as a whole…
In the UK, age ratings for computer and video games are determined by a two-tier system that works within voluntary European guidelines known as PEGI and the mandatory BBFC regulations as determined by the Video Recordings Act of 1984.
The voluntary PEGI system is one of the most robust in Europe and, in terms of rating criteria, is the strictest. It is supported by the entire value chain from software developer and hardware manufacturer through to publisher, distributor and retailer.
So from my limited understanding of this – The British Board of Film Classification (BBFC) are legally binding, whereas PEGI only advises parents. As Ireland doesn’t have a mandatory ratings body, that I know of anyway, then the only thing to go on is the advice of PEGI. So I wouldn’t imagine that a retailer would be able to stop someone underage from buying the game.