Listen to as many singers as you possibly can - and try to sound like none of them.
You have your own voice, explore it as far as you are able and push it to its limits, then expand those limits.
Listening to the older singers helps enormously in this - you don't want to sound like an octogenarian (presuming you aren't), so you are forced to rely on your own resources.
No harm in learning Irish songs as long as you don't try to sound what you are not - many of them Anglicise perfectly - many of the English language ones probably originated elsewhere anyway.
The best advice we ever got from traditional singers is that the words are far more important than the tunes when it comes to interpreting the songs, the latter is there to carry the former.
The English language song tradition is by and large a storytelling one, if you sing narrative songs you are a singer of tales.
Niamh Parsons is an extremely pleasant and skilfully singer, but if you want to learn Bold Doherty listen to Mary Anne Carolan's singing of it first - I'm pretty sure Niamh did.
If you want a good selection of Irish singers and songs (and English), become a member and P.M. me with a contact address - I'll be more than happy to supply you with a wide selection.
By the way - there's no such animal as "an Irish voice"
Best wishes
Jim Carroll