Old Irish is a notoriously difficult language, and the chapter begins with an account of why this is so, exploring the features such as consonant mutations, palatalization, the absolute and conjunct inflections of verbs, conjugated prepositions, verb-initial sentences, and infixed pronouns that make it so distinct from the other languages considered so far. But even so, it is still of Indo-European stock, and a couple of selections from the Táin Bó Cuailnge reveal some of this shared ancestry. The chapter also offers an extended comparison with Welsh, another important member of the Celtic branch, highlighting, with an excerpt from the Mabinogion, the Celtic features common to it and Irish, as well as the points of difference between the two.