I'm flying into Ireland in Sept. 2017 and want to hire a car for ± 30 days - BUT I want to take it to Wales on the ferry then to London and Southern England and return it to Gatwick on Oct 10? Can't seem to find a car hire company in Ireland that allows that?
It does not surprise me that you cannot do an international car hire like that. I would recommend dropping the Irish car off near the ferry and hiring another on the other side.
If you are going through Wales only to reach southern England I would consider using the train to get to Southern England and rent a car once you get there.