It was the first significant engagement of forces following the cessation of arms agreed in the previous year between the Irish leader Hugh O'Neill and the English Crown commander, the Earl of Essex.

In the course of a two-week assault the English troops established a garrison near Armagh, taking heavy casualties, and Mountjoy retired with difficulty to Dundalk.

In September 1600, Mountjoy moved north from Dublin and concentrated at Dundalk, in order to mount an expedition further into Ulster and re-establish a garrison at Armagh, which position had been evacuated by the English Crown forces after O'Neill's victory at the Battle of the Yellow Ford in 1598.

The Moyry Pass (or "Gap of the North") was the sole point of entry to Ulster (much of the terrain being wooded and mountainous), and it had been well fortified by O'Neill with trenches and barricades.

Taking advantage of a misty day on the 25th, an officer named Thomas Williams (who had commanded the Blackwater Fort during the Battle of the Yellow Ford) made a sortie into the pass.

Mountjoy retired to Dundalk - on either 8 or 9 October - but on the 14th word reached the English camp that O'Neill had abandoned the Pass and retreated to a crannog stronghold at Lough Lurcan.

The most likely explanation for O'Neill's withdrawal from his position of strength is that he was short of ammunition and food and feared a flanking attack on his rear from Newry.