William Hays was born and raised in Ireland, and came to America with his mother and grandfather when he was about 16 years old. His mother being a widow???

check this out:
http://cotton.homeip.net/Home/Gedfiles/n_1f.cfm#6 updated June, 2012 [old link: http://cotton.homeip.net/home/notes/not0037.html]

William Hays took part in the siege of Boonesborough, having been enrolled as a pioneer soldier of Kentucky on June 10, 1779, in Captain John Holder's Company, at Boonesborough. (b)
"Filson Club Publications", Vol. 16, p. 255

434. . . William Hays, who was killed by JAMES DAVIS on Femme Osage creek, in 1804

433William Hays took part in the siege of Boonesborough, having been enrolled as a pioneer soldier of Kentucky on June 10, 1779, in Captain John Holder's Company, at Boonesborough. (b)
"Filson Club Publications", Vol. 16, p. 255

From 1779-1783, he was a captain at Bryan's station under Col. Levi Todd and Col. Daniel Boone. An excerpt from Lyman C. Draper's interview with Daniel Boone's nephew, Daniel Bryan, in April 1844, gives this account of Captain William Hays:
"Captain William Hays (Col. Daniel Boone's son-in-law) raised a party of about 12 men on Kentucky River, a dozen miles from Lexington, probably the second day of the siege and pressed on to Bryan's - found a fence built across the lane - partly torn down - not his cross fence, but the lane fence, and leaped over it into the field. The Indians were posted each side of the lane, for some twenty paces, and as Hays' party of horsemen dashed down the lane, they raised such a cloud of dust as to greatly screen them from the enemy's aim; and hence probably it was that they were not all cut off. Hays was wounded in the neck and came near falling from his horse, but escaped." Capt. Hays detailed to attend to the building of canoes and collecting of provisions for Gen. George Rogers Calrk's army in 1781.

In the fall of 1784, Daniel Boone, his sons-in-law, William Hays and Joseph Scholl, and their families settled on Marble Creek, north of the Kentucky River and about five miles from Boone's station. About 1785 William and Susannah Hays came into possession of Daniel Boone's Marble Creek Farm where they lived until they moved with Boone to Upper Louisiana in 1799, William Hays and William Hays, Jr. going with the group which went overland through Lexington, Louisville, Vincennes to St. Louis.
From the Spanish authorities William Hays received land grant number 1670, containing 510 acres, near the Missouri River in the Femme Osage District. Susannah Boone Hays lived only a year after their arrival in the Louisiana Territory, and her husband survived her by only four years.
ref: "The Boone Family", pp. 116-7, 632, 635

433Susannah Boone, the oldest of the four daughters of Rebecca Bryan and Daniel Boone, was born November 2, 1760. She died October 19, 1800 in what is now St. Charles County, Missouri and was buried in a little family cemetery on Spanish Land Grant 1670, in the Missouri River Bottom, now a part of the experimental farm of the University of Missouri. All evidence of the cemetery, where later her husband, too, was buried, has long since been destroyed by the many floods which submerged this land (This information about the Hays Cemetery was obtained from some of the older settlers of nearby Defiance, Missouri)
ref: "The Boone Family", pp. 116-7, 632, 635