I can't offer much in experience in the usual Archipelago map, but for whatever reason, I have experience in isolation. Especially in Archipelagos.
Let's say we are isolated. A few more islands are discovered, but that's it. Because we are playing as Ragnar, however, we have a few advantages/disadvantages. Here's my strategy:
Settle all the islands. This may sink the economy short-term, but since Ragnar's Financial, we can still tech at a good pace and keep research (for the most part) above 50%. Plus, there are tons of resources. These are key in an isolated Archipelago, since there's less land and whatever exists is usually spaced out.
The Berserkers, for the most part (unless my knowledge of the promotion path is screwed up, which it probably will be), are NOT nerfed. Thankfully, so. Galleons need Astronomy, and enable trading across Ocean. However, the fac that we may be isolated does pose a problem: To transport troops, we need Galleons. To protect the Berserkers in the Galleons, we need Frigates. To get Frigates, we need Chemistry, whose prereq is Gunpowder. Which means that the ability to build Berserkers may be gone. Again, not sure here. If this is the case, build all the Berserkers you can before Gunpowder. If not, here's a new problem: Metals are somewhat scarce in Archipelagos. Not always, but sometimes.
Teching. This isn't as much of a problem as it normally is, since we're Financial, but if we're isolated, well....