In general an Irish resident is liable to CGT on their worldwide gains. It's irrelevant whether they bring the proceeds of the transaction back to Ireland or not.
If the OP's parents realised a gain each time they sold and rebought, then there will have been a potential liablity to Irish CGT.
Under the Ireland/Spain Double Taxation Agreement, if they paid Spanish CGT in relation to the transaction, that is allowable as a full credit against any Irish CGT liablity. So it's possible, in fact, that their Irish liablity, allowing for the credit, was nil.
But it's also possible that it wasn't. In which case there is now a liablity for unpaid CGT, plus interest and penalties, in relation to one or both of the previous transactions.
Exactly the same considerations apply to the current transation except, presumably, there's as yet no danger of interest and penalties.
I agree with Brendan and Clubman; time to get a professional adviser. The advice they offer may be welcome, or it may be unwelcome, but either way it's advice the OP's parents need to hear. Plus, a professional adviser will be best positioned to assist in minimising whatever liablity there is.