Complex sentence: On July 10,1688, in a clandestine ceremony, he married Katherine O'Brien, daughter of Henry O'Brien (styled Lord O'Brien) and his wife Katherine, in her own right Baroness Clifton.
Split into simpler sentences: On July 10,1688, in a clandestine ceremony, he married Katherine O'Brien. She was daughter of Henry O'Brien (styled Lord O'Brien) and his wife Katherine, in her own right Baroness Clifton.