I've not seen earlier use of 'Planksy' than that in the Neals 'A Collection of the Most Celebrated Irish Tunes', Dublin, c 1724. The facsimile edition, 1986, has notes by Nicholas Carolan, and that for "Planksty Plunket" where the word first appears, goes (part of note only): "The precise meaning and derivation of the term 'planksty' is unknown, but it indicates a lively instrumental air." Nicholas Carolan is at the Folk Music Society of Ireland, whose homepage you can click on from my website.