Theory 3: Robert Fitzgerald, the younger

The Fitzgeralds objected to Thomas, Earl of Desmond marrying Catherine, daughter of William McCormick of County Limerick, Ireland. Thomas resigned his title and estates to his younger brother and fled to France, where he died in 1420. He left his two sons, Maurice and John Fitzgerald, of whom the first had a son, Robert, who returned to Ireland and took the name of Adair.

Yea, But
Robert Adare was living in Galloway, Scotland as early as 1380. This was almost 100 years before Robert, son of Maurice, could have been born.