I'd say Siobhan was middle-aged too @Sittingontheveranda. I grew up in N. Wales. Niamh wasn't unheard of, but the welsh version, Nia, was more usual.
I went to a Welsh school and Carys was a very popular name.
Names that were very popular, as in there were several of them in a class, were Siân, Bethan, Carys, Rhian, Eleri, Nia, Sioned, Delyth and Dylan, Dewi, Rhys, Bryn, Gareth, John, Siôn, Huw. They would be in their 50s now.
Carys wasn't as popular in South Wales, but Cerys was.
A Carys younger than about 45 is likely to have a welsh connection but not be from a welsh-speaking home.
If you like the name, use it , but be prepared to get asked if there's a Welsh connection. At least, of the Welsh names, it's easy to say and spell, and is quite nice.