Geirmund's Hall is very easily found from Ivarstead- the burial mound in town is off in the right direction, and the Geirmund Lake island is visible from atop the domed structure. Geirmund's Hall itself isn't actually a structure- the burial mound is within a cave that is located on the island in a position from which it faces Ivarstead.
You can pick up a Nirnroot from the edge of the island before entering, as well as a couple of batches of Bloody Crown mushrooms at the cave entrance itself before going in.
The entrance is a narrow tunnel that quickly opens out onto a ledge- this ledge overlooks much of a small well-lit chamber lined with various mushrooms. Also in the cave are a pair of skeever that you'll need to take down, though this should be a negligible bump in the road at this point.
In case you managed to arrive at Geirmund's Hall before finding any other parts of the Gauldurson Brothers' questline, an adventurer lies dead in this initial cavern as well with a copy of Lost Legends in his hands. This will, of course, start the quest for you if you haven't already.
The last major feature of the initial cavern is a large pit- this is your way down into the burial mound itself, and you should be careful you don't fall in- there's simply no easy way back out at all. Once you head down the pit, you're committed to completing the dungeon, so gird your loins, poison your weapons, and down the foods for any long-term buffs you want to have as you go in.
The pit is deep, but you'll need to hop down the middle of it fearlessly- there's enough water at the bottom to cushion your landing, but if you cling to a wall you stand the risk of hitting the stairs or ledge that lead out of the water and breaking a leg instead of coming down safely.
The pool of water at the bottom is plenty deep enough to swim in, and while the West side has some stairs up out of it to a door, the East side has a ledge where you can grab a coinpurse and the contents of a burial urn. Before you head up stairs and onto dry land, though, submerge yourself and look to the South. An underwater passage lies here, with a now-unspringable spear trap in it. Take a quick breath and duck down it at top speed and you can reach the far end, where a treasure chest waits. A burial urn or two also sit in the dirt and gravel at the bottom, but if you're low on breath these aren't really worth the time- just take the chest contents and return to where you can breathe. Don't forget to take some Hanging Moss clippings where they dangle into the water from the top of the hallway's arch either.
