I fancy trying to write a lament but am wondering in what time signature a lament should be and how the notes are grouped. i.e. jigs are 6/8 2 groups of three notes. 
Generally, there is none. Some laments do sort-of follow some time signature, but there is no single one. Other laments as played have no discernable time-signature at all.
Then again, there are lament-like passages in classical music that of course are written down; but it’s probably much, much easier to just listen to a recording, or send someone a recording.
Caj
O’Connell’s Lamentation, my current practice effort, is written in 3:4
My advice is, don’t worry about it. Unlike jigs or hornpipes or reels, laments and other airs can be written in ANY time signature (granted some will “feel” more appropriate than others…a “lament” in jig time would “feel” a bit too happy for the subject matter, I’m thinking). Work out your tune and THEN figure out what time signature it happens to fall in (note: Sometimes you’ll find yourself writing in multiple time signatures, and that’s fine too).
Redwolf