They came out of Irish folk through both of their parents and cite Mary Black as a biggest influence. They played Irish folk prior to getting big and enfused it throughout their albums. To the point where when they released an album full of Irish folk songs, Home, it felt like a natural transition and could have been a regular release.
I get the comparison to 90s girl-folk/country pop, but I feel like you're defaulting a little because they happen to be guitar fronted pop songs. There's a massive variety there.
And they're not the most authentically Irish music, no, and that's because they're shamelessly pop. They're not striving to be Christy Moore.
Let's not forget that country music in itself is birthed in Irish and Anglo folk tradition.