If you want to try something a little different and if you like world influences and electronica try Ecova. This is a trio with members from France,(displaced Californian) Iran, and Algeria. I find it very very cool. I have Heaven's Dust and Soft Breeze and Tsunami Breaks. If I remember correctly Soft Breeze is the straight stuff and the other in the club mixes. Both nice in their own way.
If you like Jazz try Madelain Peroux or Cassandra Wilson
If you like Country There is always K.D. Lang and Emmylou Harris.
If you like Blue Grass try Allison Krause or Kate Wolf.
If you just want some great female vocals try Sarah Mclaghlin "Mirror Ball", Bonnie Raitt "Road Tested", or Jennifer Warnes "Famous Blue Raincoat" or "the hunter".
Ok now I'm rambling. I really just got on here to say I agree with you, Sinead has an awesome voice and I love her early releases.