Aine, I've seen these besides fireplaces to keep your kettle or pan warm. There were round, square and oblong ones like yours, and heater-shaped ones to take sad irons or coal irons as well. There's a pic of some neat 1940s ones here:
http://en.wikipedia.org/wiki/Trivet but they were made over a long period (200 years) and still are, so I don't know how old yours might be.
To clean off the old brasso I seem to recall that alcohol works, the rubbing sort from the chemist rather than the hard stuff! Or try coca cola, that brings brass up very clean, but don't leave it in too long or it eats through the brass. I've seen brass stuff that was unrecognisably black brought up clean by an overnight soak in coca cola. (Just think what it does to your innards!)
