They could have originated from England, via the lowlands of Scotland known as the Borderlands. I did a quick lookup and the Hemphill's, as you may know, are said to come from Northern Ireland. The Hemphills are known to have originated in Southwestern Scotland in an area near the Borderlands. With the Lowland Scotish links I've seen this to me means they were Scots-Irish or as we Americans would say...they were Scottish Irishmen. Most likely your Hemphill's were Presbyterian that were moved to Northern Ireland to colonized Ireland in the 1600s becoming what was called Ulster-Scots. These were not Irishmen, they did not speak Gaelic, nor where they apart of the Celtic awakenings of the early 1700s that became so popular in the 1800-1900s. It was illegal for Roman Catholic, Gaelic speaking Irish to immigrate to the Colonies prior to the late 1700s.
The Irish Rebellion of 1641 saw the massacre of nearly 4000 Ulster Scots by the Irish, and about 12,000 were expelled from their homes. In 1642 an army of 10,000 Scottish soldiers arrived in Northern Ireland to quell the rebellion and fought there until 1650. In 1690 a new wave of Lowland Scots emmigrants arrived in Ulster to escape a famine. By the early to mid 1700s the Ulster Scots began migrating to America, mainly to the mountains of Pennslyvania and from there they migrated South following the mountians towards the Carolinas. Mainy more went directly from Northern Ireland to Charleston S.C.
Whether or not your Hemphills followed this migration pattern I don't know but thousands of Scot-Irish emmigrants did.
Read Albion's Seed by David Hackett Fischer for a good understanding of the Scots-Irish.