This is Einar's helmet and it's a copy of the 10th century helmet which was found in Gjermundbu in Norway. This particular helmet was excavated with chainmail protection attached to it but only at the back.
A helmet with chainmail attached to it completely like you see here was found in Valsgarde in Sweden but it is dated roughly 200 years before any "real" viking activity.
This helmet is very effective on the battlefield where safety is concerned.
The book "Die Wikinger" by Nurmann, Schulze and Verhülsdonk (isbn 3-932077-01-6) shows these helmets on page 28. The book is also available in English.