Where does “mat” come from?

mat (Old Irish) comes from Old Irish masu, from Old Irish má, from Old Irish ís, from Proto-Celtic ɸīssu, from Proto-Indo-European pedsú, from Proto-Indo-European pṓds, from Proto-Indo-European póds, from Proto-Indo-European ped- — to walk, to step; to stumble, to fall.

mat (Old Irish): third-person plural present subjunctive of masu