The route I suggested has the benefit of being easier riding as France is much flatter in the west. You don't need to cycle directly along the coast to enjoy the benefits of flatter geography.

Your goal is to get to Ireland, so going for a tour through Spain won't be very helpful, unless you want to take a ferry from Spain to Ireland. if there is on. If so, then I definitely recommend biking the Camino de Santiago! I did that two years ago, and the Camino Bike Route is FABULOUS!