I live in British Columbia, Canada but was trying to connect with Johnstons in/near Magherafelt. I love Ireland.
I didn't know my grandmother's siblings other than a one time meeting with her sister Rose who was in her 70s or 80s at the time. So your mom may not know her grandmother's siblings. But I have 3 brothers for Margaret Jane from Andrew's second wife Rachel Margaret Lennox. And I have traced Joseph, William, Wallace and Annie from the first marriage from birth to death with rumours that there was also a Margaret and a Bessie. I wonder if there would be a Margaret from each marriage. I would be thrilled if your mother could fill in any blanks. Maybe she has old photos, letters, whatever. I know that Andrew Johnston's father was Joseph but I don't know his mother's name (suspect Elizabeth) and I don't know if Andrew connects to the other Johnstons in Magherafelt as he was born Co. Armagh and resided near Coagh in his first marriage. Here's hoping your mom can recall some things about the family tree.
When did your grandfather Samuel die?