ISB were known as a "psychedelic folk music" group, went through several incarnations and continued sporadically till a few years ago. They also played at Woodstock - the real one - and appeared on a DVD, but their song was cut from almost 10 minutes to just over 2...IMO, they were too eclectic for the promoters there...
The Incredible String Band was popular among a certain group of "hippies" in the US, generally us east coasters. They were vastly more popular in Britain and Europe.