The Kilkenny Confederacy sent 2,000 troops in three regiments under the command of Alasdair MacColla to support Montrose's Royalist army in Scotland who were fighting against the Covenanters in 1644.
For example, in July 1644 Colonel William Sydenham defeated a Royalist plundering party from the garrison of Wareham at Dorchester, and hanged six or eight of his prisoners as being "mere Irish rebels".
[13] One example of the severity of this law was the massacre of some Welsh civilian camp followers (who were mistaken for Irish) by Parliamentarian soldiers after the Battle of Naseby in 1645.
As he explains, "The 'laws' of war evolved like any primitive legal code, from the principle of reciprocity; self-interest counselled against brutality if there was the chance of being paid back in the same coin".
The pitiless execution of Covenanters by Mac Colla's followers would seem to show that for the Irish, too, battle against British forces was waged without moral restraint.
For example, O'Neill, immediately after Benburb, sent 150 prisoners (excluding officers, whom he kept for ransom) under escort back to Scottish quarters (Hogan, war in Ireland)".
Plans to try him for treason, for bearing arms against the king, were dropped when the Parliamentary side threatened to retaliate in kind, and he was exchanged for a Royalist officer.