Can I please ask you to clarify what time period it is you're talking about? From your phrasing, it isn't very clear whether you're making a reference to the kind of slavery St Patrick is described as being subjected to, or the use of Irish indentured servants in the Americas as a labour force by the British.

If you're asking about the practice of Irish indenture being used by the British in their Caribbean and American colonies, then you should be aware that scholars overwhelmingly reject the suggestion that this was a form of slavery. Slavery is not synonymous with unfree labour; it is a particular form of it. Irish people (and others) taken to the New World were taken as indentured servants, not saves. These answers on the topic might interest you to that end (from our FAQ); this old answer of mine gives a brief summary of the topic and links out to other, more detailed answers I've given in the past.

Edit: /u/sowser's name in central Texas is cursed / when I find out / he pooblish first. (Same first line and same reference to Saint Patrick!)

Pre-Reformation: there were no anti-slavery groups, except that the Church was a bit negative on slavery, and it dwindled in the British Isles. This is from the FAQ: How extensive was the slave trade in pre norman Ireland, by /u/textandtrowel. He mentions "Only later did it become considered un-Christian to enslave women and children taken in war." (though right after quoting William of Malmesbury circa 1066 detesting the practice). He points to further writings by John Gillingham.

If you're referring to some modernish Irish slavery, say, Reformation or later, the usual assertion is that Irish were chattel slaves in the Caribbean, and it's recent claptrap much supported by neo-Nazis. I think this recent reply is a good place to start as well as the links that /u/sowser posted: What was the reality of the Irish slaves in the 18th century in America?, because flaired /u/freedmenspatrol points to previous posts, on the reality of the situation and why it has recently become popular, and to a Web page of someone's meta-work on it.

By chance, as we say in Middle Earth, this got referred to today: What happened to the Roman system of slavery after the fall of the Roman Empire? Was the legal basis of 18th-19th century slavery derived from Roman law, or a completely separate system?.

Specifically, /u/textandtrowel and /u/Tfish addressed pre-modern attitudes towards slavery, with a bit from Saint Patrick. Summarized, "I've never seen reference to any premodern complaint against slavery as an institution."