As you no yourself 'fadas' on Irish names as vital. If lost, it can totally change the pronunciation. That's why the fada on the 'o' in Róise is really important. It elongates the o as in 'Rooooo-sha' or Roisín (Ro-sheeeeen).
Either way she'll be fine, if other Irish names can make it this one can too!
If you're bringing that up cause I didn't use them its cause I'm too lazy to find it as it isn't on my keyboard , which might be another issue with any name that has accents that aren't on a standard keyboard outside of your country. I've learned to live with my name outside of my home by using two anglicised (sp?) names and even they end up butchered and they don't have fadas or accents so what can you do. Pick a name you love and realise they all have their limitations. For example she may grow up and head a business section whose main office is in hong kong, whatever name you give her will have difficulties then. So I say don't stress over that and choose what you love. You do seem to love Róise .