A Catholic army of united Irish clans from the Wicklow Mountains led by Fiach MacHugh O'Byrne and James Eustace, 3rd Viscount Baltinglass of the Pale, defeated an English army under Arthur Grey, 14th Baron Grey de Wilton, at Clan O'Byrne's mountain stronghold of Glenmalure.

Despite this, Grey was inexperienced in waging war on Irish terrain and had been previously out of active military service for twenty years.

Grey had also received little information on how Ireland was to be governed, even after being appointed Lord Deputy, and so his task was seen as a purely military affair.

However, Grey had been under strict instructions not to give any such protection or pardoning, and perceived this message as indicative of the arrogant behaviour of the Irish rebels.

[5] When what appeared to be the vanguard of the Irish rebels presented itself before Grey, he ordered his forces to engage immediately, against the advice of some of his more experienced Kildare allies.

English losses included Peter Carew, cousin of his namesake colonist who had made claims to, and won, large tracts of land in southern Ireland.