As The Red says England is just too great and Ireland too meh.
England (or more specifically southern England) is just so much better land than the rest of the isles. This gives us the situation where even if somehow the Irish do come to conquer England (somehow) then they would likely end up with their core lands around London.
To have a independent, rich, successful Ireland is difficult but doable. To have it as a UK level super power though is just ASB without utterly changing the definition of Ireland.