Nothing to do with singers, really. For many years, the many thousands of Irishmen who volunteered to fight in the British Army during the first world war, were regarded as traitors by SOME of their compatriots. The context of the song can remind some of "perfidious Albion". The attitude is slowly changing.

Regards

p.s. That said, there is certainly a long tradition in Ireland, in both her languages, of taking a localized political interpretation of songs that may not have had that in mind at time of composition!