GRMA, a Aonghuis. I've a student named Rebecca who's been wondering what her name might be in Irish.
That site lists
Ribeacá, though I've also seen
Ríobhca elsewhere.
Ríobhca wins the Googlefight by a wide margin:
http://www.googlefight.com/index.php?lang=en_GB&word1=r%EDobhca&word2=ribeac%E1