There is a section of the FAQ devoted to this question, which you can access here.
Two of the major threads discussing the question that have appeared on this subreddit can be read here and here, with discussions led by u/sowser and u/agentdcf.
In addition, a great resource addressing the "Irish slaves" meme as a whole, with an emphasis on the situation in the Caribbean - which exposes it as the ahistorical, white-supermacist-inspired internet propaganda it is - is the work of Liam Hogan, which he has recently gathered together in an accessible way here.
^I see you been pointed to an FAQ about it but I'll explain too if you'd like from my perspective.
Basically the question doesn't really boil down to 'irish' rather that it boils down to Catholic Irish. Many British politicians and generals had dual heritage, Wellington was Irish and was an MP there for a time before he became Prime Minister. Obviously Irish people were not treated as slaves.
19th century is far too late also. In 1829 Catholic emancipation was passed allowing Catholic Irish MP's to sit in the House of Commons and as the Fenians and IRB nationalist began to grow and commit terror attacks more attempts were made to give Ireland home rule by various U.K. Prime Ministers with the most famous being William Gladestone. In fact throughout the latter part of the 19th century the Irish MP's (many catholic) held 80 seats and basically dictates which party (Liberal or Conservative) would get into power through an alliance with them.
Also generally in the 1800's many Irish also made up the armed forces.
So no they were not treated as slaves at all for the 1700's and 1800's but that doesn't mean they were treated as lesser. They is a degree of 'racism' toward Catholic Irish, and they were seen as uneducated peasants not worthy of equal rights. You can see that the British didn't value them in their response to the famine. Also 'racism' against Irish carried on for a long time only really ending in the 1990's. I'm sure people have wrote more detailed ways they were used as 'slaves' in terms of labour but in general there was no policy to treat all Irishmen as lesser humans although they did with many Catholics.