By the Spring of 141, Governor Urbicus had uncovered much about to mysterious land of Hibernia. For a century the Roman merchants in Britannia had engaged with the local Celts, who were eager to give away gold, copper, and especially slaves in exchange for what the Romans considered cheap raw material. When Rome decided to change the nature of its relations with Hibernia in 141, this trade backfired spectacularly. Brittonic villas were full of Hibernians from nearly every tribe and region, many of whom were persuaded into informing the Romans about their knowledge of island. As the seas calmed, the navy got to work, Roman worships and marines demolished every raider outpost on Hibernia in less than two weeks, clearing the way for the army. Unlike the Caledonians however, the Hibernians were unanimous in their opposition to Rome, they knew what nightmare awaited them if captured, they'd spent 100 years inflicting it on one another.
Given the populace had been feuding for so long, their forces were highly experienced, but still typically lacked the equipment and discipline of the legions. Nonetheless, hearing what happened in Caledonia, along with their own first engagements with the Romans, the Hibernians decided a direct, decisive battle wasn't the best strategy. Indeed after the first skirmishes on the coast, Urbicus described the interior as looking like every human had vanished the day before. Hovels and hillforts were still surrounded by well-weeded crops, pigsty's were freshly emptied, it was clear the locals had run. The interior of the island was heavily forrested, scarcely settled, and virtually impossible to move supply wagons through, the perfect place for ambushes.
As the three legions assigned to secure Hibernia advanced beyond the coast, hit and run attqcks became the regular business of the day. In particular the Celts would try to isolate single cohorts, and pelt them with arrows and slinger bullets, withdrawing when help arrived. While Hibernians were being killed, this was mostly done by auxiliary cavalry and missile troops, not the legions themselves. Only a handful of legionaries would be killed or wounded in each attack, but these happened several times a day across the island. Additionally, foraging, which was needed to supply the troops far from the ports, required entire centuries to venture out together to avoid ambushes. In the worst case, a single isolated cohort lost 2/5 of its strength in a 16 hour long night attack on its camp.
The Romans soon responded in turn, as one by one, the stumbled upon the camps of the secluded locals. Entire tribes were annihilated overnight, the men fighting to the last, leaving their families to be enslaved. As the Dacians had a generation ago, many Hibernians killed themselves and their families to avoid the wrath of the legions. In an attempt to flush out the enemy, the Romans started efforts to burn the forests whereever they could, difficult given the wet conditions, but somewhat effective. By mid summer, with food supplies beginning to run low, some groups gave the Romans their surrenders. The terms were harsh, most of these Hibernians were forced onto subpar land they didn't own, forced to start crops months late, and given essentially no assistence. By autumn famine was setting in, what aid arrived was more aimed at stopping rebellion than stoping hunger. It's estimated that by the end of 141 that as many as 75,000 Hibernians were dead, nearly a third of the population, having taken about 8,000 romans with them.
While there had been some serious losses of veteran legionaries, a problem which persisted into 142, Urbicus and the Emperor were satisfied, All of Western Europe was now under the eagle. However, holding the islands would prove difficult. Two legions were still required in Hibernia by spring of 142, and the other three in Brittania. Nonetheless, the new Imperial province of Hibernia was created in June of that year. With the last of the Hibernian resistance starving out by early summer, the two visting legions were returned to their frontier posts on the continent, and three legions with their equal number of Auxilia held the two great islands, even as tempers flared among the newly conquered. The celebration of the creation of a new province would be short lived however, as all eyes would soon be fixed on the Rhine...