As times change, so does music. Rock and Roll is a product of the twentieth century that has carried over into this century and shows no signs of slowing down. It does evolve. So has Celtic folk music. Many new bands are taking the old music of their ancestors and giving it new life by blending it with the music they grew up on.
Nobody knows for sure.
It can't really be pinned to one band. Back in the 1960's there were probably a few bands roaming around. The people of Scotland and Ireland always like to get together for a bit of fun. There were probably a few bands of younger people that would play a little rock during a town festival. They would then put up the electric guitars and pick up a fiddle or pennywhistle and pump out some jigs and reels.