Britain did not go to war to defeat fascism
It is true de Valera achieved political power democratically against the fascist movement in Ireland but Britain, on the other hand, did not go to war in 1939 against fascism as such.
Churchill had been, notoriously, an admirer and friend of Mussolini and he declared that a British Hitler might be necessary in certain circumstances. Britain went to war against its former ally in 1939 not to defeat fascism as such but to assert its position as the predominant world power. Other world powers, such as the US, remained neutral until their own interests were at stake.