Complex sentence: The younger Jenico married Maud Plunkett, daughter of the first Baron Killeen and had one daughter and heiress Margaret who married firstly Sir John Dowdall of Newtown, and secondly Rowland FitzEustace, 1st Baron Portlester.
Split into simpler sentences: The younger Jenico married Maud Plunkett, daughter of the first Baron Killeen. He had an only daughter and heiress Margaret who married firstly Sir John Dowdall of Newtown, and secondly Rowland FitzEustace, 1st Baron Portlester.