Where does “dáu” come from?

dáu (Old Irish) comes from Proto-Celtic dwau, from Proto-Brythonic dọw, from Proto-Celtic *duwo, from Proto-Indo-European dwóh₁ — two.

Ancestry of “dáu”, step by step

StepLanguageWordMeaning
1Proto-Celticdwautwo
2Proto-Brythonicdọwtwo
3Proto-Celtic*duwotwo
4Proto-Indo-Europeandwóh₁two
Every word from Proto-Indo-European dwóh₁ →Every word from Proto-Celtic *duwo →Every word from Proto-Brythonic dọw →