There’s some debate as to how old Tynwald Hill is, and what it originally represented. The web site Isleofman.com has this to say about the origins of the hill:
The mound is nowadays thought originally to have been a burial mound of the Bronze Age. Similar sites were found throughout the Scandinavian lands and are said to have been raised as altars to the god Thor. From religious sites they developed into places where people of a community gathered together.
The hill has never been excavated, so nobody really knows. A burial cist was discovered near Tynwald Hill (50 yards to the north) and some people believe that this (not the existing hill) represents the site of a mound, which was re-used as the original parliamentary meeting place. Presumably this theory places the existing hill within the Viking period.