Britain and US offered condolences for Stalin

It was a grossly insensitive act by the Taoiseach, even if the extent of Nazi-perpetrated genocide and war crimes wasn’t known in April, 1945. But de Valera was following established protocol for the death of any head of State whose country had a legation in Ireland.
In de Valera’s defence: 1) Ireland was a non-combatant in World War Two; and 2) despite our neutrality, de Valera allowed the quiet return of captured Allied airmen who crashed in Ireland, while detaining Luftwaffe pilots. He secretly let Allied aircraft avail of the Donegal corridor, and useful military intelligence was passed to the Allies throughout the war.