I am the Grand-daughter of Jemima Hunter JOHNSTON (the 9th child of Thomas Edmondston JOHNSTON and Eliza Jane Burt SMITH). Thomas was a Mason, and family legend has it that he built the house in which your Great-grand-father and my grand mother were raised at Beachpark, Baltasound, Unst.
Jemima and Annie (Anderson JOHNSTON) who were the two youngest girls in the family of 11 children sailed for Australia on a ship called the Waipara. After docking in Brisbane both girls worked for a period at Newstead House before marrying and having families.
I have another cousin who has done extensive research on the Johnston side of the family, so it may be of use to you. As time permits I will scan some photographs of the croft house (Beachpark) both in it's prime and as the ruin it is today. To the best of my knowledge Thomas Edmondston JOHNSTON (junior) (born 18th July 1867) was the last of the family to live in the house. He was known as 'Tammy' and he passed away in 1951.