McGegan, who has led the group since 2001, announced in August 2004 that he would leave at the end of the 2004-05 season.
Marwood is a member of the Florestan Trio and a regular guest soloist/conductor with the Academy of St. Martin in the Fields. A longtime collaborator with Thomas Ads, he will give the world premiere of the composer's Violin Concerto in September. He first conducted the ICO last year.
As artistic director of the ICO, Marwood will also oversee the orchestra's MBNA Shannon International Music Festival, a five-day annual event in July.
"It is with huge pleasure that I look forward to working with the Irish Chamber Orchestra as artistic director," Marwood said. "We had a wonderful time playing together last year—their energy, commitment, and refinement prove they are a musical force to be reckoned with. I know this is going to be a thoroughly enjoyable partnership."