To expedite the plot, they befriended members of prominent native Irish families (including two powerful chieftains from Ulster).

Between 100 and 400 members of families prominent in Loígis and Uí Failghe were summoned to Mullaghmast in County Kildare, under the pretext of performing military service.

A horrible and abominable act of treachery was committed by the English of Leinster and Meath upon that part of the people of Offaly and Leix that remained in confederacy with them, and under their protection.

It was effected thus: they were all summoned to shew themselves, with the greatest number they could be able to bring with them, at the great ráth of Mullach-Maistean; and on their arrival at that place they were surrounded on every side by four lines of soldiers and cavalry, who proceeded to shoot and slaughter them without mercy, so that not a single individual escaped, by flight or force.John O'Donovan (scholar), in his edition of the Annals, stated: In the year 1705, there was an old gentleman of the name of Cullen, in the County Kildare, who often discoursed with one Dwyer and one Dowling, actually living at Mullaghmast when this horrid murder was committed, which was about the sixteenth year [recté, nineteenth] of Queen Elizabeth's reign; and the account he gives of it is, that those who were chiefly concerned in this horrid murder were the Deavils, the Grehams, the Cosbys, the Piggotts, the Bowens, the Hartpoles, the Hovendons, the Dempsys, and the FitzGeralds.

Said Lalor, as he was entering the fort, saw the carcasses of his slaughtered companions; then drew his sword, and fought his way back to those that survived, along with whom he made his escape to Dysart, his family's ancestral home.