Upvote:3
The Encyclopedia Brittanica states:
The modern Irish language, which is very similar to Scottish Gaelic, was widely spoken up to the time of the Irish Potato Famine of the 1840s and the subsequent emigrations.
The Irish Language in Ireland: From GoΓdel to Globalisation probably contains all the answers you might need, including demographical figures (just look at the List of Figures) and (from the excerpts) a very detailed look on different reasons for the decline of Irish (e.g. that it never became the language of government before the 20th century - first, Latin was used, then English; or noting the pressure to learn and use English starting with Poyning's Law from 1494).