In the Second World War the Irish Free State never declared war on Germany and throughout remained a threat to British survival. The Royal Navy was denied use of Irish ports while German U-boats sailed in Irish waters to attack our life-saving convoys of food and arms. As British cities were devastated by German bombs the lights of Dublin provided a beacon for German bombers, particularly to Merseyside and Glasgow.
I met and was friends with many southern Irishmen in the British Army in the war, but it is true that most of them were mercenaries, rather than ideological soldiers.
Ottery St Mary, Devon