Complex sentence: The school was under the trusteeship of the Christian Brothers, however they withdrew from Education in Ireland in September 2006 and the school passed into the trusteeship of the Edmund Rice Schools Trust upon the Brothers' withdrawal.
Split into simpler sentences: The school was under the trusteeship of the Christian Brothers. However, they withdrew from Education in Ireland in September 2006 and the school passed into the trusteeship of the Edmund Rice Schools Trust upon the Brothers' withdrawal.