The 26-county state of today's Republic had been independent since 1922, removing the British crown from its constitution in 1936, and with the last British bases being handed over to the Irish authorities in 1938 as part of a general settlement of the six-year-old “economic war” between the two countries.

Britain could have considered conceding formal republic status, but taoiseach (prime minister) de Valera seems not to have considered it an urgent matter, declaring when asked about such a change in 1945 that "We are a republic" on the basis of the 1937 constitution: it was to be left to his Fine Gael opponents to make the status formal in 1949.

Perhaps you mean an offer of reunification of the six northern counties (retained by the UK at Partition) with what was at this time Eire. Such a proposal had been made in 1940, subject to the agreement of Northern Ireland’s Unionist government, a proviso that made it a non-starter (the northern PM Lord Craigavon had on the contrary urged a British military takeover of the south).

Imposing such an arrangement would however have triggered civil war among Churchill’s own Conservative MPs and quite possibly in the north whose Protestant majority still bitterly opposed any suggestion of rule from Dublin. Even a secret deal deferring action until after the war would have only deferred the crisis, and de Valera was himself aware of the likelihood of conflict.

Nor was Irish intervention in the war a particular priority for Britain as the immediate crisis of 1940 faded, Dublin maintaining a mildly benevolent neutrality behind its formally strict refusal to take sides: plans were even drawn up for joint resistance in the event of a German invasion of the south.

A readable introduction to wartime relations is Robert Fisk’s In time of war: Ireland, Ulster and the price of neutrality 1939-1945, which includes the joint defence planning under “Plan W”: Craigavon's letter and tentative British plans to seize the ceded ports are discussed in Ian S Wood, Britain, Ireland and the Second World War.