This is a personal numeral meaning two people.
The explicit reference to 'people' is included in the word itself. Beirt daoine would be wrong. However, forms with greater specificity such as beirt bhan (two women), beirt fhear (two men), beirt aibhléiseóirí (two electricians) and so on are perfectly correct.
In parts of Ulster, beirt is also used to refer to animals or inanimate objects. There are gramatical subtleties however. Tá dhá ghloine agam (I have two glasses) is correct. Tá beirt ghloine agam is not. But where the noun is not specified, for instance in reply to being asked how many glasses you had, beirt on its own would be correct.
The personal numbers between one and twenty-three in Irish are:
Do not confuse seisear (six people) with the similar word seisrach (a team of six horses).