From the nighttime views of the island under attack in Shell Shocked, Part 1, Berserker Island appears to be a very rocky, bare island. It is very 'vertical', having a tall central mountain with very little spread of hills of fields around it.
More detail is revealed in "Something Rotten on Berserker Island". The rocky central island is surrounded by significant sea stacks. The island has trees and grass, and a large amount of white-flowering bushes closer to the village proper. The interior of the island has a "maze of caves", in which ice structures can be found, extremely similar in appearance to Bewilderbeast ice.
Though it doesn't appear in game, Dagur mentions Berserker Island during a quest in the expansion Rise of Stormheart.