Churchill, albeit through the intervention of Neville Chamberlain, did indeed offer the possibility of a "United Ireland" in June 1940. The final memorandum on the proposal, presented to de Valera on 26 June by the diplomat Malcolm MacDonald, stated as follows:

1. A declaration to be issued by the United Kingdom Government forthwith accepting the principle of a United Ireland.

2. A joint body including representatives of the Government of Eire and the Government of Northern Ireland to be set up at once to work out the constitutional and other practical details of the Union of Ireland. The United Kingdom Government to give such assistance towards the work of this body as might be required.

3. A joint Defence Council representative of Eire and Northern Ireland to be set up immediately.

4. Eire to enter the war on the side of the United Kingdom and her allies forthwith, and, for the purposes of the defence of Eire, the Government of Eire to invite British naval vessels to have the use of ports in Eire and British troops and aeroplanes to co-operate with the Eire forces and to be stationed in such positions in Eire as may be agreed between the two Governments.

5. The Government of Eire to intern all German and Italian aliens in the country and to take any further steps necessary to suppress Fifth Column activities.

6. The United Kingdom Government to provide military equipment at once to the Government of Eire...

According to MacDonald, de Valera understood these terms to mean that "Eire was to enter the war immediately, but a United Ireland to be a deferred payment." And assuming that the United Kingdom even won the war (which seemed in doubt at the time), the "payment" might not come at all, since it depended upon the assent of the government of Northern Ireland. Finally, and perhaps most problematically in de Valera's mind, Ireland would have to forsake its policy of neutrality. He emphasized these concerns in his official response to Chamberlain delivered on 5 July:

The plan would involve our entry into the war. This is a course for which we could not accept responsibility. Our people would be quite unprepared for it, and Dail Eireann would certainly reject it... The plan would commit us definitely to an immediate abandonment of our neutrality. On the other hand it gives no guarantee that in the end we would have a united Ireland... The only way in which the unity which is so needed can in our view be secured is [...] by the immediate establishment of a single sovereign all-Ireland Parliament, free to decide all matters of national policy, internal and external--the Government which it would elect being responsible for taking the most effective measures of national defence. I regret that my proposal that the unity of Ireland should be established on the whole country becoming neutral is unacceptable to your Government. On the basis of unity and neutrality we could mobilise the whole of the manpower of this country for the national defence...

With this, Chamberlain ended further negotiations. He wrote to his sister shortly thereafter: "The real basic fact is that it is not partition which stands in the way at this moment but the fear of Dev and his friends that we shall be beaten. They don't want to be on the losing side..." Curiously, however, Churchill himself went on to make another offer for Irish unity in late 1941. On 8 December, immediately following the Japanese attack upon Pearl Harbor, de Valera received this rather cryptic message:

From Mr Churchill to Mr de Valera. Personal. Private and Secret. Begins. Now is your chance. Now or never. 'A nation once again.' Am very ready to meet you at any time. Ends.

Churchill's intentions here remain uncertain, but his vision for the process of unification does not seem to have departed fundamentally from Chamberlain's in 1940. Again, nothing came out of the proposal. De Valera reached the conclusion that "it was Mr Churchill's way of intimating 'now is the chance for taking action which would ultimately lead to the unification of the country'... I saw no opportunity at the moment of securing unity, that our people were determined on their attitude of neutrality."

Sources: Tim Pat Coogan, De Valera: Long Fellow, Long Shadow (London: Arrow Books, 1995), 551-553; Brian Girvin, The Emergency: Neutral Ireland 1939-45 (London: Macmillan, 2006), 96-135, 291; Eunan O'Halpin, Defending Ireland: The Irish State and Its Enemies Since 1922 (Oxford: Oxford University Press, 1999), 181f.