@AnnaBlush
@KirstenBlest is right when she says the fada lenghtens the vowel sound.
The correct spelling is Róisín, but you're right too, fadas are often left out, especially outside Ireland, but within Ireland as well. People still just pronounce the name as if the fada or fadas were there (hopefully).
For example, the name Seán has a fada on the a, but it's often left out and the name spelt Sean. Strictly speaking, without the fada it should be pronounced differently, as Shan. It also has a different meaning - old - instead of being an Irish form of John. But if you met a fadaless Sean, you'd call him Shawn as if the fada were included in the spelling. Leaving out the fada can be considered an anglicised spelling of the name, I suppose.
I'm a fan of fadas personally, and for me Róisín doesn't look right without them, but it's easy for me as I live in Ireland, and I understand why people leave them out too. Sometimes people use them on birth certs etc to get it right there, but are more relaxed about their use day to day. For what it's worth, in 2021 there were 9 Roisins, 4 Róisins and 152 Róisíns born in Ireland.