The name Menna can be Welsh or Cornish and used as a surname or forename.
I could only find a meaning for the cornish version which is closely linked to Welsh. I'll keep trying to find out for you as I live in Wales. In cornwall the name derives from Meneth meaning a tall hill or mountain. It also is linked to place names Menna in St. Dennis and Ladock. Both places were once called Meneth. I'll ask a welsh speaker about the welsh version of the name and get back to you.