One further thing, when we bought fuel in Waterford to depart to Inishmore, we were charged VAT at 23%. Departing from Waterford to Popham we weren't. That brought the price down to £1.76/litre. The best way then seems to be fuel up before you go, or land somewhere like Haverfordwest to refuel, then fill up on your way home.
That must be a mistake. It has never been possible to fuel up in Ireland VAT free and duty free for private flights were removed a few years ago.