Thanks to both contrinutors so far for the information.
This prompted me to look at Aer Lingus's web site, to see what their policy was.
Not expecting to find a reference to GPS at all, I actually found this:
Permitted in-flight but not during taxi, take-off, initial climb, approach and landing: Various devices listed, beginning with "Laptops with CD ROM or DVD drive" and including "GPS handheld receivers".
There's a further section on devices prohibited at all times, which begins "Devices transmitting radio frequency intentionally".
My initial thought when posting was that use of my GPS was possibly contrary to legislation. I didn't think to look for airline policy. It seems that I was compliant with that policy.
Although not surprised that the CC didn't know what the device was initially, I would have thought the conversation with me and subsequently the Captain would have cleared it up.
I'm still interested to hear what other aviation professionals think about this. I will be writing a polite letter to Aer Lingus now - At the time, I felt quite humilliated in front of the other passengers, as it appeared that I was doing something dodgy and had to be chastised. Not a nice feeling!