Complex sentence: Only two of the dioceses, Diocese of Down and Connor, and Diocese of Dromore, are entirely within Northern Ireland, the other 3 dioceses and the Archdiocese of Armagh straddle the border.
Split into simpler sentences: Only two of the dioceses: the Diocese of Down and Connor, and the Diocese of Dromore, are entirely within Northern Ireland. The remaining three dioceses and the Archdiocese of Armagh straddle the border.