In the fifth year of the war Agricola, himself in the leading ship, crossed the Clota, and subdued in a series of victories tribes hitherto unknown. In that part of Britain which looks towards Ireland, he posted some troops, hoping for fresh conquests rather than fearing attack, inasmuch as Ireland, being between Britain and Spain and conveniently situated for the seas round Gaul, might have been the means of connecting with great mutual benefit the most powerful parts of the empire. Its extent is small when compared with Britain, but exceeds the islands of our seas. In soil and climate, in the disposition, temper, and habits of its population, it differs but little from Britain. We know most of its harbours and approaches, and that through the intercourse of commerce. One of the petty kings of the nation, driven out by internal faction, had been received by Agricola, who detained him under the semblance of friendship till he could make use of him. I have often heard him say that a single legion with a few auxiliaries could conquer and occupy Ireland, and that it would have a salutary effect on Britain for the Roman arms to be seen everywhere, and for freedom, so to speak, to be banished from its sight. 

Tacitus: Life of Agricola 1:24