It looks like a misspelling/mistyping of '
Israel'; in fact, I misread it as
Israel at least twice.
Even Google gives the 'Showing results for
Israel', but if I say, 'Search instead for Isael', it does of course come up with results, including a Brazilian midfielder named Isael da
Silva Barbosa.
According to ebabynames.com (which also comes up on my Google search), Isael is in fact a form (Portuguese) of
Israel. It does not, however, offer any clues as to the pronunciation beyond that. :/ I'd guess, given the
Israel connection, that your 'ee-sa-EL' guess is not too far off, though it might be closer to IZ-ay-əl or IS-ay-el (I really am not an expert on usual Portuguese pronunciations, though!).
I'm guessing a lot of people would mistake it for
Israel or even (presumptuously) assume it's a typo and attempt to "correct" it to
Israel which could lead to a lot of frustration to anyone named Isael. (I have a brother named
Aidan who has endured a lifetime of people mispronouncing and misspelling his name, often with no logical reason, even when reading the name off a computer printout!) I don't mean to say that I think this is a good reason not to use the name. People (like me) can get used to seeing/hearing things they're not initially used to - how else can we ever learn? ;)
The name is quite lovely. It has a sort of "softer" feel to it than
Israel, I think, but it doesn't exactly sound "feminine" to me.