Interesting stuff. I have always viewed the Roman occupation of Britain as expensive and unproductive imperial overreach. Caesar kicked it all off because he was pissed that Celtic tribes were sailing over to participate in the Gallic Wars.

No Roman effort ever really pacified the locals and this was all happening with the backdrop of political instability back home ranging Caligula to Nero to Commodus until that guy was killed by Russell Crowe (actually strangled in the bathroom by his gladiator buddy Narcissus) . The Roman authorities has countless failed incursions into the north over 400 years with little to show for it. Those Roman soldiers probably felt a lot like our boys trying to ride herd on Iraq or Afghanistan.

Given all that, I can see how they left Ireland to be.

I dunno, the Romans managed to pacificy a large chunk of Britain after a while. While never putting an end to the Pictish problem, southern Britain was a huge agricultural boon for the northern parts of the late empire. Additionally, with the conquest the Romans managed to now control most of the world's tin supply, critical for bronze production.