That's what Hypertext Webster (http://work.ucsd.edu:5141/cgi-bin/http_webster) says:

Planxty \Planx"ty\, n. [Cf. L. plangere to mourn aloud.] (Mus.) An Irish or Welsh melody for the harp, sometimes of a mournful character.

So it seems Barbara and Murray are right, as the L stands for Latin.
Andreas