I think its easy to forget about how little the Romans actually knew about their empire and what lay beyond it. When we see the perfect maps drawn up by modern historians, we imagine that that was the way that the Romans saw their world- but of course it was not. Tacitus reveals this when telling us about Agricola in one telling incident. Agricola wanted to conquer Ireland and add that to Britain as part of the empire- in part his reasons were prudential (the British were less likely to revolt if they could not see freedom across the Irish sea) but they were also strategic. Tacitus's description of the strategic reasons to conquer Ireland though make clear how wrongly he imagined the Atlantic Archipelago. He thought
Ireland, lying between Britain and Spain, and easily accessible also from the Gallic sea might serve as a valuable link between the provinces forming the strongest part of the empire (Agricola ed. Mattingley 1970 para 24)
However great a historian he was, Tacitus was no geographer! Tacitus was also no idiot- he would not have written this down in a hagiography if he did not beleive it and why should he not believe it- all he or Agricola knew was that Ireland was westward of Britain, its extent and position were not noted (particularly as the Romans were coastal and not oceanic navigators). Tacitus's short statement about the strategic advantages of annexing Ireland makes clear the degree to which beyond the boundaries of Rome, the Romans were ignorant of what they faced.