William was the brother of John Mercier McMullen, who emigrated to Brockville, Ontario, Canada in 1848. John M. was married to Sarah Charlotte Nesbitt. It is highly likely that Margaret (wife of William) and Sarah Charlotte were sisters, but I have no proof.
John Mercier McMullen, a noted author, produced what he titled an "Historical Sketch of the McMullen Family," in 1902. This remained unpublished until 1981, when a great great granddaughter, Ralyne Elayne Westenhofer, published it herself.
In this work, John M. states that his wife was the daughter of William Nesbitt of Castlecomer. He also mentions that his wife's mother was "the widow Ward." When the McMullens arrived in America in 1848 they visited Sarah Charlotte's brother (probably half-brother) Robert Ward in Jamestown, NY.
There is quite a bit on his brother William in the Sketch. William and John were sons of John McMullen (ca 1779-ca 1857) and Mary Anne Mercier (ca 1781-May 1854), both of whom died in Kells, Co. Kilkenny.