On New Year's Day 1577, a brutal massacre of the Irish gentry by Sir Henry Sidney's troops took place at Mullaghmast in County Kildare. Sidney invited all of the Gaelic leaders of Laois and Offaly to a peace conference at Mullaghmast. They arrived unarmed and were slaughtered with their families by the English soldiers surrounding the castle. Estimates of the dead range from 40 (the number of Gaelic lords there) to hundreds. O Mordha vowed to avenge the deaths of his countrymen. He kidnapped two men called Harrington and Cosby, relatives of important people.