Yes, the Celtic Irish did some farming, but the the man in the street, err, bog, was far more likely to be a herdsman. The farms (monastic communities, perhaps, excepted) tended to be clustered around the port cities nearest England, or around the seats of Viking and later Norman Lords. Farmer was a low social position to the Celts, who aspired to be warriors, bards or brehons (judges).
Farmers were more valued by foreign invaders, as they were easier to exploit. A herdsman who has taken his flock, and family, twenty miles up into the hills, to an ill defined summer pasture, can, unless watched constantly by armed men, vanish with all his kin and belongings. A farmer has far less ability to stuff his oat crop into a sack and disappear with it. The farmer, and the fruits of his labor, will be easy to find when the new foreign lord's tax/rent collectors show up. The herdsman, not so much. Farming, far from being what the Celts preferred, is what they were forced to, by the destruction of their society, by the Vikings, Normans, English and Lowland Scots.
Of course, given that you have posted that following herds is so useless an existance that it justifies a farming people in exterminating the herders, in order to take their land, I hardly expect you to admit my point.
Your opinions regarding the ancient farming and herding practices of the Celts show how desperate you are to justify the continued suppression of Irish independence. Your statement that the Celts were mainly a herding people is factually incorrect.