The idea of toxins fits in well with the poisonous yew from which the best bows are made. And it also fits well with the 'noxious vapours' that ancient writers attributed to the cause of diseases, especially plague. All makes good sense in its own way. What's also interesting is that this section of the zodiac also holds other poisonous creatures, the snakes of Ophiuchus and the Scorpion. Its also been suggested that the Celts had a boar constellation in the region of Libra-Scorpio, which also fits as the boar is reputed to have poisonous bristles on his spine.
But for me the weirdest thing we've had mention of here, is the ogham with Sagittari, genitive of Sagittarius. This isn't a name as we should expect on an ogham stone, its a type of person, an archer, a station in life. It does become a sort of name when applied to the constellation. But what on earth is it doing in the depths of Ireland?