I'd say
Bord do thriúr 'a table for three people'.
Bord triúir would seem to indicate 'a three-person table'.
https://www.teanglann.ie/en/fuaim/Triúr
Of course the pronunciation would be altered when triúr is lenited.
Triúr originally comes from trí fhear > tríear > triúr.
_________________
Is Fearr súil romhainn ná ḋá ṡúil inár ndiaiḋ
(Amhlaoibh Ó Súilleabháin)
Please wait for corrections/ more input from other forum members before acting on advice
I'm familiar with Munster Irish/ Gaolainn na Mumhan (GM) and the Official Standard/an Caighdeán Oifigiúil (CO)