He at some point had an affair with Vera O'Drake. This would result in the birth of Rumpus McFowl,an illegitimate son. He later married Downy O'Drake, either a sister or a cousin of Vera, who became the mother of three of his children. Those are:
In 1885 the Clan's hereditary lands would have been seized due to Fergus' inability to pay his taxes. But Scrooge spend his savings at the time, 10,000 dollars, to pay the taxes and became the new owner of their lands. While Scrooge was away Fergus and his family moved back to the clan's castle, abandoned for centuries, in Dismail Downs. The family continued to work to pay for the taxes and Scrooge sent them some of the money he earned while traveling.
Fergus became a widower on 1897. Gideon McDuck, the Italian character who has been used for decades as Scrooge's brother, was presumably born some time after that as an ilegitimate son of Fergus.
Scrooge became rich in the Klondike and returned to Scotland in 1902 as a bilionaire. Scrooge's intention was originally to settle in Dismail Downs but he quickly changed his mind and decided to settle in the United States. He wanted to take his family with him. His sisters accepted but Fergus decided to stay. He died during the night and his three children left Scotland.