I have been able to research this surname to Ireland but need help from this point. My great grandfather x 3 was Andrew Johnston, 1801-1946. Andrew Johnston was born and died and is buried in County Antrim, Belfast, Ireland.
He was a minister, probably of the Presbyterian faith. He died about three weeks before his youngest child, Margaret, was born in 1846 (at the time of the "potato famine"). Andrew was disowned by his family for having married a milk maid
His wife was Martha Davidson who was born 31 July, 1801 also of Belfast but came to America with her son, daughter in law and newborn grandson, Andrew, via Glasgow Scotland. She died in Clayworks, Iowa.