Complex sentence: O'Connor first became involved in politics in 1991 when he was elected to Dublin County Council, becoming a member of South Dublin County Council when it was created in 1994, of which O'Connor was also a member until 2003.
Split into simpler sentences: O'Connor first became involved in politics in 1991 when he was elected to Dublin County Council. That authority changed to South Dublin County Council in 1994, of which O'Connor was also a member until 2003.