Hugh Dubh's father, Art Óg O'Neill, was among those exiles who made careers for themselves in the Spanish Army of Flanders.
O'Neill's men became the nucleus of the Ulster army of Confederate Ireland - a de facto independent Irish state.
Hugh Dubh was captured early in the war by Scottish Covenanter enemies, but was exchanged back to his own side after the Confederate victory at the Battle of Benburb in 1646.
Hugh Dubh was subsequently released into Spanish custody on condition that he would not serve in campaigns against English forces.
He did not, therefore, return to Flanders, but was posted to Spain, where he became a General of Artillery, helping to suppress a rebellion (known as the Reaper's War) in Catalonia.