Mr.Prendergast quotes some contemporary authorities, which seem to be decisive on this point.

In the same year was published by 'G.S., minister of God's word in Ireland,' 'A Brief Declaration of the Barbarous and Inhuman Dealings of the Northern Irish Rebels ...; written to excite the English Nation to relieve our poor Wives and Children that have escaped the Rebels' savage Cruelties.' This author says, it was the intention of the Irish to massacre all the English.

On Saturday they were to disarm them; on Sunday to seize all their cattle and goods; on Monday, at the watchword 'Skeane,' they were to cut all the English throats.

The former they executed; the third only (that is the massacre) they failed in.
That the massacre rested hitherto in intention only is further evident from the proclamation of the lords justices of February 8, 1642; for, while offering large sums for the heads of the chief northern gentlemen in arms (Sir Phelim O'Neill's name heading the list with a thousand pounds), the lords justices state that the massacre had failed.

Many thousands had been robbed and spoiled, dispossessed of house and lands, many murdered on the spot; but the chief part of their plots (so the proclamation states), and amongst them a universal massacre, had been disappointed.
But, says Mr.Prendergast, after Lord Ormond and Sir Simon Harcourt, with the English forces, in the month of April, 1642, had burned the houses of the gentry in the Pale, and committed slaughters of unarmed men, and the Scotch forces, in the same month, after beating off Sir Phelim O'Neill's army at Newry, drowned and shot men, women, and priests, in that town, who had surrendered on condition of mercy, then it was that some of Sir Phelim O'Neill's wild followers in revenge, and in fear of the advancing army, massacred their prisoners in some of the towns in Tyrone.