Mairead is a form of Margaret. So I don't see any problem if there is a Rosemarie (Marie) or Moira / Maura in the family already. Or was your concern more similar sound than origin?
You do know that Mairead has an "ay" sound on the end, right? ("mah-RAYD" rhymes with parade.)Sorry to be pendantic. Mairead is one of my all time favorite names so I'm overly protective of her. :-)