Where does “forunni” come from?

forunni (Old Irish) comes from Old Irish -ni, from Proto-Celtic snīs, from Proto-Indo-European nō̆s — we.

Ancestry of “forunni”, step by step

StepLanguageWordMeaning
1Old Irish-ni1st person plural emphatic suffix
2Proto-Celticsnīswe
3Proto-Indo-Europeannō̆swe
Every word from Proto-Indo-European nō̆s →Every word from Proto-Celtic snīs →Every word from Old Irish -ni →