古爱尔兰语是最古老的爱尔兰语，为一种或多或少可从其相关数据重建的一种古老语言。它的时间约为6世纪到10世纪，并在之后派生为中古爱尔兰语。

古爱尔兰语首次被发现在一些6世纪的拉丁文宗教手稿的页边空白处。除此之外，有很多早期的爱尔兰文学经典如11世纪的《牛皮书》（Lebor na hUidre）和12世纪的《伦斯特集》（Book of Leinster） ，虽然都是在中古爱尔兰语的时代记录的，但他们在特性上，实际上都是古爱尔兰语。

值得注意的是，虽然古爱尔兰语是现代爱尔兰语、苏格兰盖尔语、和曼岛语的原型，但它和后三者分属不同且独特的语言。概括地说，古爱尔兰语比它的后裔拥有更多的屈折变化，在语音及文法结构上也有很大的不同。

Old Irish (Old Irish:Goídelc) (sometimes called Old Gaelic) is the name given to the oldest form of the Goidelic languages for which extensive written texts are extant. It was used from c. AD 600–900. The primary contemporary texts are dated c. AD 700–850; by AD 900 the language had already transitioned into early Middle Irish. Some Old Irish texts date from the 10th century, although these are presumably copies of texts composed at an earlier time period. Old Irish is thus the ancestor of Modern Irish, Manx, and Scottish Gaelic.