Saw Louis Killen just a year or so ago, he must be well in there for longevity...
Packie Manus byrne as mentioned, still going strong...
Longest running roup with no changes? One candidate would be the Clutha, been running over 40 years and the only change is that (the fabulous) John Eaglesham no longer performs with them.
Another one might also be Flora McNeil of Barra, active at singing and collecting when Hamish henderson and Alan Lomax went collecting in the early 1950s.
What about Sheila Stewart? I'd wager like most traditional singers, she was singing from a very early age indeed and is now not quite so early-aged..
Bob Davenport must be another who's been round a while.
Then there's the wonderful John Milnes Baker from Connecticut, under appreciated, under-heard and just turned 75. told me a fabulous story last month when I visited of being in Moe Asch's store on Union Sq. and meeting a man with a guitar and wiry hair who was waiting for the start of the Sunday backroom session but was early and struck up a few songs..So John, in 1947 or so, sat alone in the shop, and was entertained by Woody Guthrie..