You're right, TerriM, that it comes from the Irish word for a swaying or rocking motion. The word can be spelled langar OR longar. In Irish, you could say:

'Tá sé ina langar (or longar)' - literally translated as 'He is in his swaying'

OR

Tá sé ag a langar (or longar)' - literally translated as 'He is at his swaying (or he's swaying)'

-- Áine