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Let's say that the Romans conquer and assimilate Ireland as they did Britain or find a Irish king whom they help unite Ireland under the condition that the Irish kingdom has to pay a small tribute to the Roman government. Fast forward a few centuries in the 400s when the last legions are pulled out of Britain and the remaining Romans are left to fend off the Germanic invasions on the island stands as a bastion of Roman civilization untouched by the barbarians. Could a possible Roman rump state be founded in Ireland or near the region's around Dublin or would they have been quickly overthrown by the Irish client state and the barbarian's in Britain?
(Note: Sorry if I missed any important details out on Roman Britain while trying to describe this, my knowledge of Roman Britain is far shorter than I know about most other provinces)