I vaguely remember a similar thread. There is another theory about the origin of the word 'Planxty,' far removed from the Latin theory of a lament, because Planxtys were used as complimentary tunes.

The Irish word Planncadh (Dineen pg. 845) can mean playing the harp. Tí/tigh is the genitive/dative case of teach- the Irish word for house. Put it together and you get

Planncadh tigh+Irwin (The harp playing for the/of the house of Irwin) which could easily be corrupted to Planxty Irwin