As it's an island, you'd usually say working "on" the Isle of Man, but the general principle is fine.
In Gaelic, you're typically "in" an Island.
That said, unless there's something funny about the Gaelic for Isle of Man I'm not getting, this should say "ann an Eilean Mhanainn".
That's what I thought -- I've commented on another sentence with anns an Eilean Mhanainn in it.
Thanks for flagging! It has been changed.