Irish, Manx and Scottish Gaelic form the Goidelic languages, while Welsh, Cornish and Breton are Brittonic.

SIL Ethnologue lists six living Celtic languages, of which four have retained a substantial number of native speakers.

[5] The other two, Cornish (Brittonic) and Manx (Goidelic), died out in modern times[6][7][8] with their presumed last native speakers in 1777 and 1974 respectively.

Revitalisation movements in the 2000s led to the reemergence of native speakers for both languages following their adoption by adults and children.

According to Ranko Matasović in the introduction to his 2009 Etymological Dictionary of Proto-Celtic: "Celtiberian ... is almost certainly an independent branch on the Celtic genealogical tree, one that became separated from the others very early.

A controversial paper by Forster & Toth[60] included Gaulish and put the break-up much earlier at 3200 BC ± 1500 years.

However, since the 1970s the division into Insular and Continental Celtic has become the more widely held view (Cowgill 1975; McCone 1991, 1992; Schrijver 1995), but in the middle of the 1980s, the P-/Q-Celtic theory found new supporters (Lambert 1994), because of the inscription on the Larzac piece of lead (1983), the analysis of which reveals another common phonetical innovation -nm- > -nu (Gaelic ainm / Gaulish anuana, Old Welsh enuein 'names'), that is less accidental than only one.

The interpretation of this and further evidence is still quite contested, and the main argument for Insular Celtic is connected with the development of verbal morphology and the syntax in Irish and British Celtic, which Schumacher regards as convincing, while he considers the P-Celtic/Q-Celtic division unimportant and treats Gallo-Brittonic as an outdated theory.

[46] Stifter affirms that the Gallo-Brittonic view is "out of favour" in the scholarly community as of 2008 and the Insular Celtic hypothesis "widely accepted".

Schumacher (2004, p. 86) had already cautiously considered this grouping to be likely genetic, based, among others, on the shared reformation of the sentence-initial, fully inflecting relative pronoun *i̯os, *i̯ā, *i̯od into an uninflected enclitic particle.

[65] Irrespectively, some scholars such as Ringe, Warnow and Taylor and many others have argued in favour of an Italo-Celtic grouping in 21st century theses.

Examples: The lexical similarity between the different Celtic languages is apparent in their core vocabulary, especially in terms of actual pronunciation.

There is often a closer match between Welsh, Breton and Cornish on the one hand and Irish, Scottish Gaelic and Manx on the other.