Peggy,
Robert Mawhinney’s father was William who appears to have lived in Grange. Occupation weaver.
https://www.irishgenealogy.ie/files/civil/marriage_returns/marriages_1869/11434/8200202.pdf
Robert Johnston is listed in Griffith's Valuation (1862) as having plot 14a (just over half an acre). The property was beside the Quaker Meeting House (gone now, but there is a plaque on a farm across the road to record where it was). His wife appears to have been named Rose. Robert's death:
https://www.irishgenealogy.ie/files/civil/deaths_returns/deaths_1889/06127/4749786.pdf
Probate abstract from the PRONI wills site:
Probate of the Will of Rose Johnston late of Millquarter Toome County Antrim Widow who died 22 March 1906 granted at Belfast to John Kennedy and James Hamill Farmers.
Abovenamed Rose’s will is on the PRONI website. Her husband was Robert and the will names her daughter Mary Ann Mawhinney or Johnston.
Rose in 1901 census:
https://www.census.nationalarchives.ie/pages/1901/Antrim/Ballyscullion/Millquarter/941425/
Tradition was to marry in the bride’s church. In Mary Ann’s case that was Ahoghill 2nd. So that was perhaps the family church. It has records from 1835/36. There’s a copy in PRONI in Belfast. (Personal visit required to view them).
The Ballymena Observer of 10.10.1879 reported that a woman named Johnston had been arrested for the murder of her brother in Millquarter. This would appear to be the relevant death for Joseph Johnston:
https://www.irishgenealogy.ie/files/civil/deaths_returns/deaths_1879/06500/4871519.pdf
Perhaps related to your family? Or maybe just a common name in the area.