Firstly, I didn't say "the Irish did it", I said that the break-up of the British Empire was a process that can be identified as beginning a minimum of two decades before the Second World War. (That runs the risk of teleology, of course, and it would be better to say that through the establishment of the Dominions and the Irish Home Rule crisis we begin to see a reconfiguration of the British imperial system towards a less centralised and at least theoretically more democratic form, and that for various reasons both structural and contingent this process of reconfiguration collapsed and produced the outcome that we eventually saw, but at any rate this process can be identified as beginning long before Hitler even began glancing speculatively at the Polish border.)
Secondly, if it is the war alone to which we can attribute the breakdown of the empire, then why didn't Scotland or Wales or the Six Counties or Nottinghamshire or Stoke Newington break away after 1945? It seems pretty obvious that, while the war was a factor of tremendous importance, there were other factors at play.