weird hybrid Gaelic-Polish facts

After the Flight of the Earls, Arthur Chichester, the Lord Deputy of Ireland, rounded up a bunch of 'woodkernes' and 'free swordsmen' (mostly men who had been clients of the Gaelic lords) and sent them off to join the Swedish army. He claimed to have sent 6,000 'bad and disloyal offenders' to the wars in Sweden, though modern estimates are lower. Many died, or deserted or mutinied along the way, some making it to the Spanish Army of Flanders. Hugh O'Neill claimed in a letter to the king of Spain that ships were deliberately scuttled, to ensure none of them returned, though this seems unlikely (he was right about how few ever made it back).

After they arrived in Sweden, many began to object to fighting against the Catholic Poles in the service of heretics, and encouraged by their priests, 1,200 or more deserted to the Polish side over time (including at the battle of Klushino). The Polish pay situation was abysmal (which was rather typical of seventeenth century armies) and after some time time with no money some Irishmen deserted the Polish army as well. A few of them then made it to Rome, looking for help from Hugh O'Neill. Others however remained in Polish service, and there were Irish involved for instance in the 1617/18 invasion of Russia.

I read about this in Jonathan Bardon's 'Plantation of Ulster', but the best source I've read is this 2013 paper in Archivium Hibernicum. Definitely see if you can get access, through the online account, or your library or whatever.