My understanding is the Margaret McClure who married John White was John Scott McClure's aunt, his father James' sister. James lived at Rathcavan as then spelt, and his daughter Mary, Mrs John Thomson, continued there after his death said to be in 1865. James' daughter Margaret (Peggy) emigrated to Victoria where she married Thomas Usher. They later moved to Warrnambool where he was a bootmaker.
James' other sister Jane married John Glasgow and lived in the townland of Clough. Two of her sons were missionaries to India - James McClure Glasgow and Adam Dickie Glasgow. Both retired to Antrim and James stayed there. Adam emigrated to New Zealand, where there are descendants, at least one of whom moved to Australia and was in Sydney by the 1980s. A third Glasgow son Robert emigrated to Victoria in the mid 1850s and also settled in the area around Warrnambool, where John Scott McClure and his brother James had moved from the Melbourne area early in 1853. Other central Antrim families settling in that area included Whites, Adams and Esler.
Johhn Scott McClure's brother James, the receipient of the potato blight letter (there were later letters), came to Victoria with his sister Margaret in 1844. James married Eliza Empy/Impey on 1st July 1844, noted in Georgiana McCrae's Journal (despite being listed as "her young man William"). Eliza was maid to Georgiana and had given short notice as "William" was obliged to take up a contract for timber-cutting in the Dandenongs (ranges to the east of Melbourne). The 'Courier' 1849 journey brought further McClure siblings William (with second wife and the two children from his first marriage) and Barbara, who later married her cousin John White, and died quite young after their first child. John White returned to Antrim about 1861 with son James, married Maria Esler, bought land on the 'Carnloagh Shore', and was buried at Broughshane, Upper Roughan on 26th April 1886.
A footnote to James' 1844 arrival was that Eliza Empey was one of the folk meeting the ship at Port Melbourne and she was the first person he spoke to. He is said to have asked about going to John McClure's. She said she was going there! However the John McLure who was tutor to Georgiana MacRae's chidren in Richmond had come from the Isle of Skye! Two of the same name in Melbourne established less than eight years - the other in Heidelberg/Bulleen area.
We are indebted for most of this information to two grandchildren of James McClure 1818-1910: the late Miss Rachael Jeffers (1900-1970, a schoolteacher who visited Antrim about 1958, and her older sister Miss Kathleen Empy Jeffers who lovingly retained her sister's collection of family history after Rachael's death.