I was surprised to see that Aer Lingus specifically list a handheld GPS receiver as equipment which can be used during the cruise, in their inflight magazine. (Surprised, since an ordinary radio receiver is not allowed). I've used it many times, and never been challenged.
On other airlines, I always ask, and policies differ. I've got the go-ahead with Icelandair and Ryanair, but have been turned down by cabin crew on a few others (where I suspect it may have been too much trouble to really check with the flight deck).
Enjoy the trip - I suspect you'll find it more interesting than usual!
Shamrock 602