There are three regional dialects in Modern Irish – Ulster Irish, Connacht Irish and Munster Irish – and many local variations within each of these. Each dialect contains some words and phrases not commonly found in other dialects.
They contain words and phrases that are correct but that do not occur in An Caighdeán Oifigiúil or the official written standard for Irish.
An Caighdeán Oifigiúil was first published in 1958 and was most recently issued in 2017 (pdf).
An Caighdeán Oifigiúil is used in legislation, official documents and textbooks, for example, as well as in other contexts.
Iontach!