Isibeal is the Irish form of
Isabelle. It is pronounced, "ISH-bale." I heard a BBC reporter with this name pronounce it, so I can say with confidence that it is correct.
As for Iseabeal, I'd guess "ISH-bel," but I'm not sure.
I'd say
Isobel the same way as
Isabel /
Isabelle. (I'm from the U.S.)