Complex sentence: St. Mary's City was founded on March 27,1634 by a group of 300 English and Irish both groups were Catholic during a time of religious persecution of Catholics in the British Isles.
Split into simpler sentences: St. Mary's City was founded on March 27,1634 by a group of 300 English and Irish settlers. Both groups were largely Catholic during a time of religious persecution of Catholics in the British Isles.