I don't know how to pronounce Dymphna. If I say it the way it looks it sounds like a mumps-type condition that makes your neck glands swell up. I know it's an Irish name and those are pronounced very differently than they look, but if it's pronounced Deena or Dinah, I say just use those names and not try to impress people with the authentic Irishness .
Dimity is icky. It makes me think of a simpering, scatterbrained little girl in a pinafore dress and fat pigtails and buck teeth. Also it sounds very similar to dimwitted.