Dia dhuit, a chara!
The Famine happened.
Contrary to popular opinion, the English/British never banned the Irish language, apart from rather unsuccessful attempts to stop the assimilation of English people in Ireland and piecemeal efforts to restrict to English the language of the Dublin Parliament* and the courts. The Irish language did decline fastest in Leinster, the region most under English/British control. Bilingualism was common elsewhere, with monolingual Irish speakers found only in particularly remote areas.
The Irish National School system didn’t help matters. Conceived as multi-denomination schools in 1831, the language of education was strictly English, a position given extra power by the wholehearted support of the Catholic Church, who saw Irish as the language of poverty . In this, they were supported by Irish-speaking parents, who were keen for their children to have access to the social and economic advantages associated with English speaking.
The Famine triggered the fastest decline, however. Partly, this was because those remote rural reservoirs of Irish-speaking were hardest hit; and partly because of emigration: speaking English was an economic necessity.
The Gaelic Revival which occurred only a few decades later did help to stop the rot, as it were, and after independence the Irish government made efforts to support the language and prevent it dying out. These have not been the roaring success the instigators doubtless expected. The reasons why are various, from the relative usefulness of English vs Irish, contrary attitudes to government mandates, the lack of inducements** (or inducements being inadequate), the boringness of the Irish curriculum in schools, a failure to set out the aims and provide measurable goals with scope for improvements, etc., etc.
But the critical point was the Famine, when the language lost the support of its people.
* – There’s a rather amusing tale, which may or may not be true, that when Henry VIII declared himself King (as opposed to Lord) of Ireland, the proclamation in the Irish parliament had to be translated into Irish by the one member of the parliament who spoke English.
** – When my mother was a child, there was a payment made to families who could prove they were full-time Irish speakers. A neighbour of her parents claimed this, but his wife only spoke English. So, whenever the government sent someone to check his household, his wife was sent to visit her parents, and their Irish-speaking maid stood in for her.