Complex sentence: Elizabeth (née Dwyer) and Peter Morrissey--were working-class Irish Catholics, who had emigrated to Manchester from Dublin, with his only sibling, elder sister Jacqueline, a year prior to his birth.
Split into simpler sentences: Elizabeth (née Dwyer) and Peter Morrissey--were working-class Irish Catholics. They had emigrated to Manchester from Dublin with his only sibling, elder sister Jacqueline, a year prior to his birth.