The term “Hill-Billies” is first encountered in documents from 17th century Ireland. Roman Catholic King James II landed at Kinsale in Ireland in 1689 and began to raise a Catholic army in an attempt to regain the British throne. Protestant King William III, Prince of Orange, led an English counterforce into Ireland and defeated James II at the Battle of the Boyne in 1690. A significant portion of William III’s army was composed of Protestants of Scottish descent (Planters) who had settled in Ulster in northern Ireland. The southern Irish Catholic supporters of James II referred to these northern Protestant supporters of King William as “Hill-Billies”[citation needed] and “Billy Boys”–Billy being an abbreviation of William. It is believed that the term “hillbilly” in the United States was conferred during the early 18th Century by the occupying British soldiers as a carry over from the Irish term, in referring to Scotch-Irish immigrants of mainly Presbyterian origin, dwelling in the frontier areas of the Appalachian Mountains[citation needed]. These Protestant Irish colonists brought their cultural traditions with them when they immigrated. Many of their stories, songs and ballads dealt with the history of their Ulster and Lowland Scot homelands, especially relating the tale of the Protestant King William III, Prince of Orange.

hillbilly –

“southern Appalachian resident,” c.1900, from hill + masc. proper name Billy/Billie.

Also as a type of folk music, first attested 1924.

“In short, a Hill-Billie is a free and untrammelled white citizen of Alabama, who lives in the hills, has no means to speak of, dresses as he can, talks as he pleases, drinks whiskey when he gets it, and fires of his revolver as the fancy takes him.” [“New York Journal,” April 23, 1900]