A Wind Between Worlds

The summer trip to England, Scotland and Ireland was supposed to be a 'roots' trip for the Kincaid family from upstate New York, where ten-year-old Thomas likes to do three things in particular: play baseball, read, and horse around with his dog, Grendel.
At the ruins of an Irish castle, Thomas comes across what looks like a Thanksgiving dinner napkin ring or maybe a neckerchief slide, the kind his older sister Emma wears to Scout meetings. It's not what you'd expect to find at a place like this, but someone must have lost it.
He figures there must be a Lost and Found in the caretaker's office/museum, right over there in the keep, which is the only intact part of the castle.
So Thomas picks up the whatever-it-is....