Cherbourg to Waterford, about the shortest distance between continental Europe and the Republic, would be about 300 miles.
Calais to London and back is about 200 miles.
Set off with about 90 gallons at an economical cruise rate of consumption around 50-60 gallons an hour (having made deductions for warm up, take off and climb) and you might just make Ireland. Factor in a bit of uneconomical flying, like combat, and you are going for a swim.
Cheers
Steve