The Irish language (Gaeilge) is set to get official status in the EU on 1 January, bringing the total to 23.
The European Commission says Bulgarian and Romanian are expected to get official status on the same day, when the two Balkan countries join the EU.
According to Ireland's 2002 census, 1.57 million of the four million population can speak Irish.
The commission says the EU will not have to translate all legislation into Irish, "mainly for practical reasons".
The EU will have a team of 29 translators and editors to handle Irish, as well as 450 freelance interpreter days annually, costing some 3.5m euros (£2.3m; $4.6m).
Despite the resurgence of interest in Irish, increasing numbers of students are choosing not to sit exams in Irish, the commission says. The language is compulsory in Ireland's schools.
The commission describes linguistic diversity as a "key theme" in the EU, noting that Catalan, Basque and Galician have been granted semi-official status.
If they become official the costs will probably be incurred by Spain, it says.
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