In names, usually the ヶ is pronounced が but not always. In this case it's おにがしま, Demon Island. I'm not going to repeat what Wikipedia says about it. You can see here.
Quite commonly used in place names. Wikipedia mentions Jiyūgaoka (自由が丘); there are also Chiyogaoka (千代ヶ丘) and Yurigaoka (百合ケ丘) in Kawasaki, Sakurigaoka (桜ヶ丘) in Setagaya and Midorigaoka (緑ヶ丘) in Meguro.