Irkngthand is a large Dwemer ruin located west of Windhelm and south of Lake Yorgrim. It can be difficult to find if you’re going by a map marker. You’ll need to rotate the map around to see that it’s not actually at the top of the mountain but down to the north of the mountain top.
In the courtyard of the ruin there a number of bandits and chests to loot. The entrance to the interior is part of the Thieves Guild questline and cannot be accessed until the appropriate point. This can be accessed by coming down the mountain from the other side, and climbing along a series of wood bridges to reach the entrance point.