Curiously it was Churchill who proposed the idea to to Anthony Eden after the fall of Chamberlain-Halifax. It was Edens job to covertly get the notion to De Valera. De Valera rejected the idea out of hand. Churchill was facing desperate odds with allof western and central Europe under Nazi Control.
The Germans were pressing in the desert war and the War of the Atlantic was hotting up with things looking grin for the UK. Manpower for North Africa Africa was a problem and after Dunkirk Britain had an army without weapons. Bringing Ireland into the war brought an additional dimension. The American Irish. If fighting for the Allies mean't the unification of Ireland. then Irish everywhere of any nationality could be appealed to. They wouldn't be fighting FOR Britain but FOR Ireland.
I've always found the boozy green beer sentimental Irish nationalism prevalent in this country unappealing. OK, so I secretly root for Notre Dame. Don't tell my wife, she likes Penn State.
The greatest gift the English gave Ireland was the English language. Gaelic speaking Ireland would still be an economic backwater.
I'm sure that idea would have gone over big with the Ulster Scots in Northern Ireland. The only result of this idea would have been a bloody Civil War with no guarantee that the Irish Reublic would win.