Were they British
An aspect of the disloyalty, to Ireland or otherwise, of those soldiers joining the British Army which seems to have gone uncommented;
Many of those absconding in the early 1940s would have been born in about 1920 and thus would have been British by birth
This status earned all the Irish people special rights, my mother among them, allowing them to reside and work within Britain, although they did need a special identity card. Such rights were denied those born elswhere in Europe
Since they were Bitish by birth and had the special rights many would think they should defend the country granting them. Certainly if they wished to retain those special rights after the war, as my Mum did, they would think evidence wartime service might become a requirement and so it would be prudent to serve in some way and be able to prove it. What would be clearer than military service