From Ballina, Humbert marched to Crossmolina and from there to Lahardaun. From Lathardaun he took the mountain path at Barnageeha but encountered major problems with the rough terrain-- his ammunition wagons and cannon were getting bogged down and some  had to be abandoned. The element of surprise was also lost when Humbert's forces were sighted by a local yeoman who reported his position to the waiting forces in Castlebar.
Despite these set-backs Humbert arrived in Castlebar and on the following day 800 French soldiers and 1,500 Irish rebels took up positions on Slievenark hill opposite 5,000 infantry and 1,500 cavalry under General Lake at Sion Hill one mile north of Castlebar. The majority of these forces were local yeomanry and militias from Galway, Kilkenny and Longford backed up by Prince of Wales Fensible and Scottish Frazer Fensibles. The Crown forces were armed with 12 cannon while the French had only light curricle guns. Shortly after
8 a.m., Humbert ordered 500 Irish rebels to march on the
artillery, which replied with grapeshot. Casualties were high and eventually, a retreat was ordered.
Humbert then ordered 500 Irish rebels to advance but in single file to avoid the grapeshot. At the same time two French contingents  advanced to the right and to the left. The orders were to advance on all fronts. This strategy proved successful and when contact was made  the fighting was wild and furious. Panic spread in the English ranks and they were forced to retreat to Stabal hill where they made a final stand.
Fighting lasted over an hour when a small contingent of
French/Irish,under Colonel Adouin, attacking from the rear
caused the English to drop their wepons and flee.This ignominious retreat became known as "The Races Of Castlebar"