Eirwen is a modern combination of the Welsh word for snow with the popular female name element (g)wen 'white, fair, blessed'. It came into use in Wales sometime in the 1960s/70s.
Medieval Welsh women's names are:
Angharad (ang-HAR-ad)
Gwenllian (gwen-HLEE-an)
Gwenhwyfar (gwen-HOY-var)
Gwledyr (goo-LED-eer)
Gwladus (goo-LAD-iss)
Dyddgu (duth-g'ee)
Lleucu (hlay-kee)
Gwerfyl (gwer-vil)
Tudful (tid-vil)
Myfanwy (miv-AN-wee)
Tangwystl (tang-oy-stl)
Lliwelydd (hloo-WEL-ith)
Meddefus (meth-EV-iss)
dd=th (th as in the, NEVER as in think)
There are a couple of others but not many. (I did my master's degree on this subject ;))