Looking for Johnston Robert Murray Agnes Murray George South Carolina PA

Trying to find any information about Robert Johnston and Agnes Murray who arrived in Charles Town,South Carolina (1767); "us-shipslists-pre1820.com". Robert Johnston age 16 and Agnes Murray age 10. Also George Murray appears in Charleston, SC, 1680-1830 Vol 2, Page 125,Directory of Scots in the Carolinas, by David Dobson.

Are these the same Robert Johnston and Agnes Murray who later married and settled in PA, where their first child George (Joseph) was born in 1779? Robert was born in Antrim, Ireland, 1752; Agnes born in 1755, also Antrim. George Murray, a brother of Agnes also settled in PA, first in Philadelphia and later in Allegheny County, PA.

Any help would be greatly appreciated. Would like to trace them back to Antrim, but must know more precisely the area of Antrim where they originated. Thank you.