The Goidelic languages form one of the two branches of the Insular Celtic languages, the other consisting of the Brythonic languages. They historically formed a dialect continuum stretching from the south of Ireland, through the Isle of Man, to the north of Scotland. There are three modern Goidelic languages, Irish (Gaeilge), Scottish Gaelic (Gàidhlig), and Manx (Gaelg). These all descended from Primitive Irish through Old Irish and Modern Irish.

The Goidelic branch is also known as Q-Celtic, because Proto-Celtic *kʷ was retained in this branch (later losing its labialisation to become plain [k]), as opposed to the Brythonic branch, where *kʷ became [p]. This sound change is also found in Gaulish, so Brythonic and Gaulish are collectively known as "P-Celtic". In Celtiberian *kʷ is also retained, so the term "Q-Celtic" can be equally applied to it as well, although it is not a Goidelic language, just as Gaulish is not Brythonic.
Early Modern Irish was used as a literary language in Ireland until the 17th century, and its equivalent, Classical Gaelic was used as a literary language in Scotland until the 18th century. Later orthographic divergence has resulted in standardised pluricentric diasystems. Manx orthography, based on English and Welsh, was introduced in 1610 but was never widely used.

(From Rapaire's Confidential Informant)