Whatever the legal definition of a deserter, I think in most people's minds the term means someone who is a member of an armed force who flees from that force, either to ensure their own safety, or perhaps to fight for the enemy for idealogical reasons.
To describe those who absented themselves from the armed forces of a neutral Ireland in order to go and actually fight against the Axis as deserters is tantamount to sophistry. They were neither displaying cowardice nor were they fighting for their country's enemies, indeed in de Valera's neutral state, the country in theory had no enemies.
That so many of his countrymen saw things differently is to their credit and to his eternal shame.