As a general of the Ulster army he fought Oliver Cromwell at the Battle of Scarrifholis in 1650.

In the north, the Parliamentarian/settler army met the Irish Ulster army at the battle of Scarrifholis and destroyed it.

The Irish Ulster army is defeated and destroyed by English Parliamentarian forces.

The battle resulted in the annihilation of the Ulster army and the loss of most of its weapons and supplies.

After this point, the Parliamentarians assumed command of the war against the Ulster army.

Over 3000 of the Ulster army were killed - 2000 on the field and another 1000 in the pursuit - about 75% of their total numbers.

O'Neill's men became the nucleus of the Ulster army of Confederate Ireland - a de facto independent Irish state.

Under his command were 1,500 soldiers from the Irish Ulster army, mostly from the modern counties of Tyrone and Cavan.

MacMahon however was a bishop rather than a military man, and at the battle of Scarrifholis he led the Ulster army to its destruction.

The Ulster army was routed and as many as 2000 of its men were killed.