Modern Sidhe

It doesn't sound like any underhill I've glimpsed, but then that's not that many. The presence of a fort sounds odd to me, like nested/redundant. But then there's a single hollow hill (which would pretty much map to fort/rath/whatever, even if, TARDIS-like, bigger on the inside), and then there's "those big realms/places you get to by going under a hill and coming out the other side"... so *shrug* Even if not Underhill, that's not to say it couldn't have been any other place somewhere in the realm of Faery, though.