the excavations on Knock Allinne near Kilcullen in the 1970's turned up mainly cattle and pig bones, little if any sheep.
Since it's a ritual site, I don't think it is necessarily representative of the agricultural reality. The impression I got from there is that the animal bones found on site were due to ritual feasting, and were predominantly cattle, with some pig.
Sheep seem not to figure as a primary food source throughout much of the Iron Age and Early Medieval period, but I think it's safe to assume they were widely kept as wool animals. There is ample evidence that wool was worn in Bronze Age Ireland [they found fragemtns of the clothing], and I think it's safe to assume that this tradition continued into the Iron Age. In fact, when reconstructing iron Age clothing, I would suggest the use of more sheep's wool than linen, since it's easier to produce and more useful in the climate and context of the Iron Age. Also, it is more typical in the Iron Age clothing of other countries, where more examples of garments have been found, e.g. Britain, Denmark, Sweden, Holland, etc.