I've just had a look in Ireland Since 1939 by Henry Patterson. According to Patterson, in 1940, Chamberlain offered "an immediate British declaration of acceptance of the principle of a united Ireland and the creation of a North-South body to work out the practical details of such a union."

There were three reasons why this offer was rejected.

The first is the commitment to neutrality. Irish public opinion was ambivalent towards Britain compared with Germany. People didn't necessarily like the Nazis, but they were far away on the continent. So an alliance with the imperialist oppressor was not appealing.

Second, were internal party and political dynamics. I don't fully understand these (at least without reading the whole chapter again), but it seems that while Fianna Fail was, of course, anti-partition, in practice many of its policies made unity difficult. For example, in 1939, de Valera said he wouldn't accept unity at the price of the project of restoring the Irish language (incorporating Northern Ireland would bring with it a lot of committed English speakers).

Thirdly, there was a widespread opinion that Britain would lose the war, so it would be a bad idea to join the losing side. Irish cooperation with Britain steadily increased as the prospects of victory increased.

Churchill didn't say he would give N.I he say "an immediate British declaration of acceptance of the principle of a united Ireland and the creation of a North-South body to work out the practical details of such a union" translation "I will think about it"

Did the republic ever own Northern Ireland? If not, how could it possibly be returned?