I would be quite suspicious about this. My granny (from eastern Europe) has a pension of about 100 euro a month. Are you sure this couple can actually afford the rent?
I would ask them for original bank statements and pension slips showing their income, use x-rates.com or similar to calculate the currency exchange.
I doubt the son's references would do much for you, since his name wouldn't even be on the lease.
If it all looks good I would still talk to a solicitor and ask where you stand legally. If they just pay the first month's rent and the deposit and stop there, where does that leave you?
That's just me, personally I wouldn't have anything against a retired couple, as long as they look after the house and pay the rent, but I'd be very suspicious. Most retired couples have their own houses and don't rent. And most of them don't suddenly leave their own country. Do they even have healthcare in Ireland?