The only possible reason I can think for the Romans to invade Ireland would be for prestige/glory of the reigning Emperor, or to distract from some debacle at home. But considering how little the average pleb in Rome would know about Ireland (or even the average Senator!) it probably wouldn't do much of that. Agricola supposedly wanted to invade, but try convincing an Emperor to fund and support an invasion of some place on the edge of the world that he's probably never heard of, doesn't know even how big it is, and has nothing valuable to take. Not many Emperors would even think twice about it.