It seems to me it was around the time of the Famine of the 1840s, given that it was particularly deadly to poorer, more rural Gaelic-speaking areas. But the best I can find are vague statements like "in the early 17th century, Ireland was overwhelmingly Gaelic-speaking" and "English had become the majority language by a significant margin by the time the Irish Free State was formed."
Can anyone find any demographic data from within, say, 1840-80 to confirm if this is in fact true?