Once you get your long-boats up and running you'll be able to take Ireland, but the Vikings are the only ones with boats that can cross the North Sea, so you can't attack their homelands in the traditional manner.

If you do really want to take jutland and hordaland it's possible, but depends somewhat upon luck. You need to send hordes of Bishops, at least one good spy, an emissary, and whatever other agents you desire in order to cause a rebellion. You could also try bribing armies, but a lot of the time the king or a prince will be in charge. The Vikings often move armies in suddenly as they are forced out of raiding territories, so it may take awhile to work, but if you can cause one of their homelands to fall to rebels and then bribe them to join you, you can then build up an army and fleet there to invade the other. Watch out again right after taking the first province though, as your forces there will probably be much weaker than the Vikings and you can't reinforce them from the mainland.


Ajax