The end of exams is a good time to set out ground rules for the future.
Don't just spring it on him, set in advance "now your exams are over we need to work out how you're going to contribute to the household if you are going to live here over the summer, can we talk about it on Wednesday" - gives him a bit of time to think about it.
He sounds like a lovely boy and you should keep things positive and friendly. It's a lot easier to do this if you introduce it immediately as soon as there is a milestone like this rather than waiting for your resentment to build till you can't face it any more.
Be clear that he's very welcome but that now he's an adult and no longer at school there needs to be a distinction between a "holiday visit" and a "living here" arrangement, and that as an adult he can't be a passenger any more.
Talk about a (theoretical) arrangement that you would be happy with if he was going to be actually living with you. I'm not suggesting that you offer this option, just make it clear that about the level of contribution you would expect from a long-term house guest. (e.g. talk about rent, contributions to food and bills, taking a fair share of the cleaning, laundry, garden and cooking chores, and giving each other personal space to do your own thing with defined - e.g. once-a-week - opportunities to spend the evening together). If necessary be more draconian than you actually feel you would want to be - the idea here isn't to negotiate an acceptable living arrangement but to convince him that living with his grandparents wouldn't be that fun in the long run and he would be better off either joining his parents in Ireland or getting a room in a shared flat.
Make a clear distinction between this and the "holiday visit" - where you are happy to cook for him and don't expect rent or chores and happy to spend lots of time with him - and say that you could only do holiday visits if they were up to a week at a time, no longer, and only once each university vacation, to give you your own space. Paint a picture of the options being occasional happy visits that he can look forward to, versus a long-term living arrangement more suited to a middle-aged lifestyle than a young man's.