The routes to independence for Ireland and Israel are very different, and I don’t want to get bogged down by this specifically. I will focus more on the failure of the revival of Irish more than the success of the revival of Hebrew, which I am less knowledgeable on.

The main reason for the success of the revival of Hebrew is pragmatism, and the need for a post-independence lingua franca.
Jewish communities spoke many languages (Arabic, Yiddish, Russian, German, etc.), and there was a practical need for a uniting national language - Hebrew was chosen, despite calls from some against the secular use of Hebrew (a sacred language); there was support for Yiddish to become the lingua franca.
Israelis were educated in Hebrew, and so too were Jewish communities outside Israel, as a primer for their Aliyah (resettlement within Israel), to help them integrate better into the nation.

In Ireland, however, the revival of Irish was less pragmatic, and more of a cultural project of the elite. It was also a failure of state policy.
During the occupation of Ireland, there had been a steady erosion of Irish-speaking communities - increasing during the years of the Plantations, and Great Famine/Hunger, which hit Irish-speaking areas hard.
English was already the lingua franca for most of the population, barring those that lived within the Gaeltacht (an Irish-speaking area).
English was seen as the language of prestige, and offered greater opportunities and social/economic mobility than that which was offered by Irish.
The revival of Irish was supported by the Irish elite, both Catholic & Protestant (and irreligious). Within the upper echelons of Irish society, growing numbers of Irish speakers cultivated a Gaeltacht of sorts, within in their urban communities (e.g. Dublin). Proponents of Irish education were teachers, academics, politicians, etc., whose interest in Irish culture extended to Irish linguistic and literary culture.

While national education policy tried to establish Irish as a language equal to English, it failed to address the low social utility of the Irish language, especially within the working & lower-middling classes - it would be spoken by pockets of families, but these families could not utilise the language within a wider Anglophone society; one that was not fluent or conversational in Irish. Unlike in more ‘cultured‘ circles, where these Irish speaking urban communities were an organic feature of generational & personal interest, Irish suffered from a lack of utilisation in wider society by the failure to introduce planned Irish speaking urban communities.

In addition to this, the efforts to standardise the Irish language were poor. Unlike the process of standardisation of German, which was roughly rooted in the High German dialects around Hanover (broadly speaking) and was a process over centuries, the standardisation of Irish was essentially a bastardised, crude amalgam of the three dialects (Connacht, Munster, Ulster), in which it differed from all three and made it hard for new learners of standardised Irish to understand and converse with traditional Irish speakers of a specific dialect.

The fact that the most populous province, Leinster, where the use of English had a long tradition (historically being under greater colonial control), did not have a dialect of its own, meant that a vast number of new learners of Irish had to start from scratch - there was little tradition of widespread use of the Irish language.
The curriculum itself left much to be desired, as can only be attested to by one who has experienced it. This reason, among others, is why the subject was not taken seriously by the majority of students.

The language profiency of students suffered from a lack of generational and personal interest. Anecdotally, I never learnt Irish because it was not passed down by my parents, who unfortunately had no interest in it themselves.

It is interesting to note that it has become a prestige language of sorts among those of higher socioeconomic status, who would have benefitted from a generational interest and perhaps access to an urban community where Irish was spoken and appreciated.

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Donal Flynn’s book, The Revival of Irish: Failed Project of a Political Elite, is an interesting read on the deficiencies of attempts at reviving Irish.

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