I honestly have no idea why this is not that well known. It's a remarkable looking waterfall with a beautiful backdrop, just a two minute drive from the ring road. It's hardly a detour and it's very easy to walk up to.
When you drive from the 1 Road into the town of Skaftárhreppur you'll get to a roundabout. Take the north exit, 203 Geirland and after about 1km you'll find the parking for the waterfall on your left, and you can park pretty close to it. It's shouldn't be more than a few minutes walk to get to the waterfall, and you can see it pretty much as soon as you get out of the car.