The Celtic family of languages were first attested in the later half of the 1st millennium B.C. in Britain and Europe. Modern Celtic languages can be divided into two branches branches. Languages within each branch are fairly close and can be understood by speakers of those languages, but the branches themselves have diverged quite a bit,
An Irish and Scottish Gaelic speaker (Goidelic) can read each others materials and have some understanding of another speaker, but have difficulties with Welsh and Breton (Brythonic). Similarly, Welsh and Breton speakers can learn to have simple conversations with each other.