This is what I find inexplicable, if Éire had indeed done such a thing it would have made little or no military sense. The gain for the British – and that’s a very important point, because it would be a British gain, not an Irish one – in terms of access to ports and airfields would have arguably been vastly outweighed by the costs in terms of weapons to protect Irish waters and airspace from aggressive attacks and putting aside civilian casualties the impact on food supplies to the UK, on labour from Ireland for British factories. And so on.
And truth is while not fully optimal Northern Ireland provided much of the strategic requirements that access to southern ports did not. Moreover the actual level of tacit and explicit cooperation in terms of air corridors, Foynes, etc etc to the British (which by the way I am strongly supportive of) was vital to the war effort and arguably could have been profoundly disrupted by the necessity for de facto protection of both their own and this island by their military forces given that our capabilities were so low.