Eirlys is the actual word for snowdrop. The first use of it as a name was in 1891 in North Wales. Looking at the data, it seems to have caught on and been fairly popular from the 20s to the 50s, which is probably why I think of it as an old-lady name; the only one I know is in her 60s, and I see it much more often in obituaries than births.
Anwen dates from 1907 with that spelling and 1913 as Annwen. It's never been as popular as
Eirlys was, so doesn't seem as dated.