I’ve been reading a lot lately about the history of the British islands in the Dark Ages. A lot is known about Viking ships from 800 A D on, which also became the standard design for a lot of other Germanic peoples. However, I have been able to find little information about ships between the Roman evacuation of Britain and the Viking era. There was clearly Celtic seamanship going on, because Saint Patrick was kidnapped by an Irish chieftan who attacked by sea around 450 A D. One account says before about 700 to 800 A D Germanic ships did not have a keel, and thus could not carry a sail. If so, were these ships rowed everywhere? I’d think even across the English Channel would be a pretty long row. Another account I have read mentioned ancient Irish using ships made by spreading hides over a wooden frame. The Celts obviously had ships because they lived on islands, but I would like to see some more sources on the internet or otherwise about Celtic and Germanic ships from 400 A D to 800 A D.
The Sutton Hoo ship, from about 625 c.e., did have a keel, so might have carried a sail;
the Nydam boat, from Schleswig-Holstien in the old Saxon part of Germany from 400 c.e. almost certainly did not.
I think you would find that most of the Anglo-Saxon migration happened in sailless rowing boats with many oars, but evidence is difficult to come by.
Currach. Look into Brendan the Navigator to find out how far you can get in one 
Dark Age Naval Power: A reassessment if Frankish and Anglo-Saxon Seafaring Activity ( http://www.book.nu/189828122X )
Admittedly, as the title suggests, there is limited information on the Celts but I suspect it is still be best source of pre-Viking North-European naval history you will find. Unfortunately my copy (published originally in the UK by Routledge) is in a crate somewhere in the UK.
IIRC the thesis was that the Vikings were only a stage in a pretty straight line of improvements, and that keels and sails and even rudder boards were available a lot earlier. The Vikings just had a dramatic land crisis that forced them to take what ship technology they had and make the most of it in seeking out new lands to colonies or conquer. Whereupon of course their knowledge of shipbuilding and navigation improved all the faster.
But you should try and get a copy of Haywoods book if you are interested in the period, it was an eye-opener to me.
This book:
Facing the Ocean, Barry Cunliffe
has a great deal of information on just this subject.
Since it’s only just outside the specified period (by about a century), it’s worth mentioning the Graveney Boat.