My understanding is that it was not a Gaelic word, and not found before its appearance in titles to O'Carolan tunes, but was attached to some of the tunes that O'Carolan composed in honour of friends and patrons. There are very few tunes called planxtys that aren't by O'Carolan. By a hasty scan through D. O'Sullivan's 'O'Carolan' it appears that all the planxtys are 6/8 time, but they are a lot of others commemorating friends and patrons that aren't called planxtys that are also in 6/8 time.