Complex sentence: Donovan was in the audience and invited Planxty to open for him on his six-date Irish tour the following week, during which their first major performance--at the Hangar in Galway--was a huge success.
Split into simpler sentences: Donovan was in the audience and invited Planxty to open for him on his six-date Irish tour the following week. Their first major performance, at the Hangar in Galway, was a huge success.