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.**

EDIT: I cannot see replies to this comment for some reason. Feel free to message me.

The short version is that Israel needed a language and Ireland didn’t.

The modern Hebrew revival movement did not begin with the Israeli government, but instead the movement’s origin can be most largely attributed to Eliezer Ben-Yehuda, a Russian Jew who died in 1922. Hebrew Revival was tied to the Haskalah, or Jewish Enlightenment, and to Zionism. All three movements were part of a larger wave of Jewish nationalism and attempts to revitalize and protect Jewish communities in the face of centuries of antisemitism, which was spiking in the 1880s in the Russian Empire, where many, many Jews lived.

In addition to this movement, many Jews already were fluent in Hebrew. Being able to read the Tanakh is crucial to a Jewish education, which in turn is crucial to a Jewish life. This Hebrew was used as a liturgical language, while Jews spoke other languages natively. Most Ashkenazi Jews spoke Yiddish (Judeo-German) and Sephardi Jews spoke Ladino (Judeo-Spanish). Jews in America tended to speak Yiddish or English, particularly the latter if they were younger. Outside of these groups, it depended largely on the nation, but dialects from Judeo-Arabic to Judeo-Malayalam and Judeo-Kurdish thrived.

In 1948, Israel was a brand-new nation dealing with an absurd amount of people. From 1948 to 1958, over a million Jews flooded into the country. 110,000 of those had arrived between May 1948 and January 1949. My source for these numbers is the Jewish Virtual Library.

From 1948-52, over 350,000 immigrants came from Muslim countries and largely spoke Ladino or Judeo-Arabic. 300,000 came from “communist satellite states”, and 33,000 from western states. This did not include the USSR. Jews from communist states largely spoke Yiddish.

By May of 1952, in the wake of 800,000 Jews being expelled from Muslim countries, 220,000 Israeli Jews lived in transition camps, 80% of them from Muslim countries. That is, quite obviously, a lot of people.

And so, Israel had many, many new residents who despised their former countries, sought to forge a tighter-knit Jewish people, and could not speak each other’s languages. The answer was obvious. Hebrew was taught to those who did not already speak it, and adapted for everyday use for those who did. The new Israeli dialect was based largely on Sephardi Hebrew, with some Ashkenazi and Yemenite influence. The government conducted a massive campaign of Hebraization, and it worked. Modern Israelis rarely speak Yiddish or Ladino, except for the old, and some Hasidic Ashkenazi communities that still use Yiddish. Many Israelis also changed their last names to Hebrew ones. To use a famous example, the Gadot family name used to be Greenstein.

Ireland, while it has a populace of people who care deeply about their population and their country, did not have major communication issues. Trying to get everyone to shift from English to Irish is much harder than trying to get a number of disparate groups to adopt a shared language that many were already familiar with.

Related to OP's question: has the Irish language revitalization effort failed? I (admittedly very far removed from this subject) was under the impression it was ongoing, but slowly working?

I don't agree that the Irish revival failed. Irish was never going to displace English as the lingua franca of Ireland because 99% of people already spoke English. This was not true of Israel and any one language -- Israelis came from backgrounds as diverse as Lithuania and Ethiopia. Ireland is a history of emigration -- Irish people were looking outwards towards Britain, the United States and Australia. Israel is a history of immigration -- Israelis were building a common identity.

If you don't consider "displacing the spoken language of the entire population" as the metric of success, Ireland's revival of Irish on its own terms is remarkably successful. It could have easily gone extinct and been lost to history (as the British colonisers would have liked). Instead it is studied by almost all students for 14 years in school. About 30-40% of the population can speak it fluently, and about 80% of the population have at least some Irish. About 8-10% of people speak it every day, and Gaelscoil are immensely popular. It's spoken by teachers and public servants. You can have a trial held in Irish. Road signs are bilingual across the country and in Irish only in Gaeltacht areas. Irish sports, music and media are a huge part of our culture and the language is very prevalent in those areas. Moreover, Irish people speak a third language (not Irish or English) at about 2-3 times the rate of other English speaking countries (75% of Irish people can speak an additional language versus 30% of British people and 20% of Americans). I believe the fact that we have a bilingual education system is a huge part of that.

You can get by without Irish but unless you're raised in the country and the culture you don't have a good sense of just how relevant it is.

Redditors ask about the revival of Hebrew a lot.

/u/ghostofherzl and /u/gingeryid (under a previous alias) have previously answered In the lead up to and when Israel became a nation, how did people develop an effective social and political state when everyone spoke a different language; or was modern Hebrew already spoken by the majority?

/u/hannahstohelit has previously answered How was the Torah used to help revive Hebrew?

EDIT: See below for an answer about Irish, this comment has reached the ping limit.

Because Hebrew was never actually a dead language. It was dead in the sense that it was nobody’s native language, but really up until immigrating to America, the existence of the USSR and within the last century, throughout history 100% of jews were able to read and speak hebrew fluently. It was really more updated than it was revived because people weren’t speaking it in daily contexts, it was used for prayer and as a lingua Franca between jewish communities, there was a lot of new things discovered in the past 2,000 years obviously and the speakers were forced into different parts of the globe so there were some differences between the hebrew in different jewish communities due to being separated (although every diaspora group always had contact with each other besides for beta israel and i think maybe bene israel as well) So, there wasn’t nearly as much work to be done for hebrew as is needed for Irish as it was just updating the vocabulary and combining the differences from different diaspora groups to create a new standardized language.