Realistically, you need the Roman Empire to be more secure, so that it can have further economic development of the West, specifically the Atlantic West. You aren't really going to see any sustainable voyages out west to ensure a reliable journey if you aren't good at sailing in the Atlantic to begin with.
So, lets butterfly away the fall of the West with a Majorian Lives PoD - lets say Ricimer gets thrown from his horse off of a cliff into an unexpectedly vicious pack of wolves well before the African Fleet would be built. This leads to the reunification and a final settlement that has a new declaration of all Foederati as citizens. Empire survives this in the West, huzzah. (Sorry, LSCat, you're going to hurt me I think for this).
This leaves us a period where the Romans can return to Britain, but furthermore, return to a province that has a surviving Dux Britannorum, making a Roman restoration much easier, to the point that the Romans are now in a position to go North once again, and also deal with Ireland for good.
A united Britannia et Hibernia effectively creates a large peaceful area that will primarily transport goods by the North Sea, Irish Sea and Atlantic. Which means trading ships can be developed that work in the ocean. Going from Ireland to Rome could be useful in a trip. If you can foster that, and through this largely peaceful Atlantic Coast in the Western Empire you can really get that economy moving.
After that, making trips westward are more possible. Find Orkney, Shetlands, Iceland, Greenland, Vinland and oh... What have we found? A perfect place for a penal colony, or a place to send troublesome characters.
But realistically, without a strong economy on the western coasts, I can't see the Romans doing it outside of an accident, let alone making the effort to capitalise on the discovery.