Manits is spot on, emphasising that polka can mean two(or more different things) Polka, as in Kerry polka, is a term used for a tune for set dancing(a form of quadrille dancing|). Not be confused with a set dance, which is a quite different kind of tune for a quite different sort of dance. And not to be confused with the dance the polka, and the tunes associated with it. Though, of course, some Kerry polka tunes are tunes used for dancing polkas to as well. Confused? you will be.
(Next discusssion: what do we mean by hornpipe?)