The Sir William Herbert who was a fellow "undertaker" with Captain Jenkin Conway was Sir William Herbert of St Julians, county Monmouth. He insisted that his daughter and heiress Mary married a Herbert and, in 1598/9, she married her 15 year old cousin. The Mary whom Jenkin Conway married was, according to Burke, the daughter of Sir William Herbert of Colebrooke.
"The Seignory of Castle Island on the death of Sir William Herbert passed to his only child Mary Herbert, who married her cousin Edward, Lord Herbert of Cherbury, a youth of fifteen afterwards one of the most eccentric thinkers and writers of his time.
"But Mary was lucky - at least so it seems. She married the dashing, young beau of the time, Edward, Lord Herbert of Cherbury, who was a member of the family's Montgomeryshire branch and thus "a cousin". Lord Herbert of Cherbury writes in his Life: "I had not been many months in the University (i.e. Oxford) but the news was brought to me of my father's death. … Shortly after I was sent again to my studies in Oxford, where I had not been long but that an overture for a match with the daughter and heir of Sir William Herbert of St. Julians was made .... Mary, after her father died, continued unmarried until she was one-and-twenty; none of the Herberts appearing in all that time, who, either in age or fortune, was fit to match her. About this time I had attained the age of fifteen, and a match at last being proposed, yet, notwithstanding the disparity of years betwixt us, upon the eight-and-twentieth of February 1598 in the house of Eyton . . . I espoused her".
"Sir Thomas Blennerhassett, and his son, Robert, obtained a part of the Earl of Desmond's large possessions, Robert m. Elizabeth Conway, dau. of Captain Jenkin Conway, by Mary, dau. of Sir William Herbert (of the Colebrooke branch of the Herberts)"