In the mid 18th century, it's estimated that about a quarter to a third of Irish immmigrants spoke Irish as their main language. As the link points out, it can be tough to discern the exact amount because it was a largely illiterate population, so there aren't huge amounts of Irish-language writing as you might expect.

Irish in the 19th century was on a major downswing, in large part due to British pressure forcing education to be done primarily in English. There was a utilitarian argument to this as well, with the general consensus being that Irish was the language of the rural backwaters, not something that would serve a speaker well. Even the great Daniel O'Connell, an Irish MP known as "The Liberator" for his work on Catholic rights, was quoted as saying,

I am sufficiently utilitarian not to regret its gradual abandonment. A diversity of tongues is no benefit; it was first imposed upon mankind as a curse, at the building of Babel. It would be of great advantage to mankind if all the inhabitants of the Earth spoke the same language. Therefore though the Irish language is connected with many recollections that twine around the hearts of Irishmen, yet the superior utility of the English tongue, as the medium of all modern communication, is so great that I can witness without a sigh the gradual disuse of Irish.

So the number of immigrants who would have spoken Irish, and especially those who only spoke Irish, dropped throughout the century, as did the language as a whole on the island.