A brief scenario idea I had on waking up...
OK, so famously the Republic of Ireland was neutral throughout WW2 despite being bombed by accident by the Luftwaffe a few times. It's unlikely they would join the war from the beginning of course. But what about later on? De Valera seems to be the main obstacle here, so let's kill two birds with one stone. Some spectacularly unlucky Luftwaffe bombers bomb Dublin by accident in May 1941 (like OTL) but manage to hit Leinster House (unlike OTL) and kill de Valera and several other TDs together with other casualties. This combines a decapitated government with public outrage and, unlike the OTL Dublin bombing (which killed 38) can't be brushed off by the Germans so easily. If anything would bring the Republic into the war, it would be this.
Now, what effects would this have on the war? The southern Irish military was obviously too small for much of a direct contribution, and in any case it would be expected to remain home to defend the Republic itself. There are some exceptions, chiefly in things like the air force where Britain had more planes than it had pilots and could always use more. Britain might supply an expanded Irish Air Force with planes in exchange for part of that Air Force being based in Britain and helping the RAF, with the other part remaining home to defend against further Luftwaffe attacks.
Ireland's entry into the war would be of more strategic importance. Britain could use the Treaty Ports again (which had been given back in 1938) which helps the Royal Navy in the Battle of the Atlantic. For that matter, Ireland would allow more anti-submarine aircraft to be based further west than OTL, which also helps protect the convoys. But perhaps the most significant effect would be on the United States: it would turn the Irish-American vote (or some of it at least) pro-interventionist rather than isolationist. Not enough to bring the US into the war itself, of course, but enough to change the tone of the debate. Joseph Kennedy would probably be placed in a particularly awkward position in London and I wonder if he might be replaced as ambassador.
Anyway, thoughts?