Finland, Poland, Belarus, Ukraine and the Baltic countries faced a long period of Russification, but their languages are still the majority in their countries. Finland was also ruled by Sweden and Norway was ruled by Denmark for a long period, and only a minority speak Swedish in Finland and no one speak Danish in Norway (except Danes living abroad). Compared to this Irish fared rather badly, with most people in Ireland now speaking English (or a dialect of it to be precise) as their main language. Why was the language shift in Ireland so much more dramatic than in the other regions facing similar forms of cultural assimilation?
That's an interesting question.
There are plenty of places besides Ireland where the local language did not do well against a competing imperial or colonial language. Gaul and Hispania lost (most of) their local languages in favor of Latin. Much of the Mediterranean coast of Africa, and the Middle East lost their local languages in favor of Arabic. Scotland mostly lost Gaelic (and Scots) in favor of English, as Wales mostly lost Welsh. In Brittany, Breton was displaced by French, and so was Provencao in the Languedoc. Venice and Genoa, which had their own languages lost them in favor of Tuscan Italian.
The Basques, however kept their language. The Catalans mostly kept theirs. The Greeks kept Greek (though many other places that had once been Greek speaking in the Byzantine Empire did not), after the Ottomans took over.
Are there some common features which determine whether languages are lost or kept?
I don't know, but maybe some others will be able to contribute more.
The Irish language declined most in the nineteenth century, more for socio economic than political reasons. Obviously English was the language of law, administration and government but it was also promoted by the Catholic church and became associated with education and prosperity. Families insisted that their children learn English to improve their prospects. The most famous example was Daniel O'Connell who spoke Irish fluently but had no interest in supporting it. This effect was true both for the state schools and non-state.
One effect of this was that nationalist politics were divorced from the language until the late nineteenth century.
The famine disproportionately affected the poor, Irish speaking population further reducing the number of speakers but also reinforced the cultural links between Irish and poverty.
While the British government promoted English it didn't do very much to actually suppress Irish. Speaking, writing and publishing in Irish weren't interfered with by law. Again the assumption was that English was an aid to progress and Irish a hindrance.