An Caighdeán Oifigiúil

An Caighdeán Oifigiúil ([ənˠ ˌkəidʲaːn̪ˠ ˈɛfʲɪɟuːlʲ], "The Official Standard"), often shortened to An Caighdeán, is the variety of the Irish language that is used as the standard or state norm for the spelling and the grammar of the language and is used in official publications and taught in most schools in the Republic of Ireland. The standard is based on the three Gaeltacht dialects: Connacht Irish, Munster Irish and Ulster Irish. In Northern Ireland and County Donegal, the Ulster dialect (Gaedhilg Uladh) is used extensively alongside the standard form as the spoken language in primary and secondary schools.

It was first published in 1958 by combining spelling reforms, which were promulgated in 1945 to 1947, with grammar standards, which were published in 1953. Revised editions were published in 2012 and 2017. Since 2013, the Houses of the Oireachtas Commission, through the translation department, has been responsible for periodic updates to the standard, with reviews at least once every seven years.