I took a jaunt through the Moore family to see if I could find any Stewarts. The title “Viscount Moore” is held by the Earls of Drogheda. It is only used by the eldest son during his father’s lifetime. The 1st Earl of Drogheda would have been Viscount Moore from the death of his father in 1643 up until his elevation in 1661.
The 3rd Earl of Drogheda’s eldest son Charles Moore married, had children, and died within his father’s lifetime. Charles Moore was born in about 1676 and died in about 1714, and I think he would have had the title Viscount Moore throughout his life. See p107 in the History of the Moore Family:
https://www.familysearch.org/library/books/records/item/595913-redirectionBurke’s Peerage lists Charles’s two sons: Henry (4th Earl) and Edward (5th Earl). The History of Irish Parliament adds another son called Charles who died in infancy. No sign of any daughters.
If there is a link to William Steuart, it may be somewhere else in the Moore family. The only Stewart I could find was Alexander Stewart (uncle of Lord Castlereagh, I think) who married Mary Moore, daughter of Charles Moore, 6th Earl (and 1st Marquess) of Drogheda. But that was in 1791.
