To expand on Bruce O.'s answer.... Planxtys can be in any time signature, but usually fall into 6/8 and are therfore played as jigs. They are a song composed for and dedicated to patron. They were usually composed on the harp, which may explain the loose time signaure. O'Carolan was a blind Irish harper in the early 18th century who composed many planxtys.

Slainte

Rick