Complex sentence: Def Leppard brought in Jeff Rich in August 1986 to play alongside Allen during Def Leppard's warm-up mini tour of Ireland, but after Rich accidentally missed a gig, he and the band realised Allen could drum alone.
Split into simpler sentences: Def Leppard brought in Jeff Rich in August 1986 to play alongside Allen during Def Leppard's warm-up mini tour of Ireland. When Rich arrived late for a gig, he and the band realised Allen could drum alone.