From studies I can only answer about the Norse peoples, and they weren't wearing socks as we would know them. It was something more akin to footwraps: https://s-media-cache-ak0.pinimg.com/236x/84/16/37/841637ad4b85f8827f38598db29bb0e7.jpg

From a view of effectiveness, they were quite good at insulating your foot, and they always fit too. These types of "socks" were used by some militaries even up to, and after world war 2. (I know of Finland and Russia using them, some even still use them.)

Edit: Spelling.

That's probably because medieval Irish garb did not usually include pants, leggings or leg coverings in general. There are some examples of individuals wearing clothing like pants, but it seems that most medieval Irish men went about with their legs uncovered. They would have worn a knee or ankle-length belted tunic with a large hooded and brooched cloak over top. The tunic's colour might vary depending on an individual's social status, but literary depictions tell us that bright white, saffron and sometimes purple cloth were used. For particularly wealthy individuals of high status, their clothing might have been richly embroidered, sometimes with gold.