Take the minor road west out of Port Ellen towards the Oa. This road is narrow with passing places. When you reach Lower Gragabus the ruined tomb is immediately next to the road on your left (south). You can park in a passing place a little further down the road.
The tomb is on the top of a rise and the road appears to have sliced off the northern edge of it. Access to the field is simply by stepping over a half-fallen old rust fence.
I thought this site was superb. The standing stone is about 4ft high and covered in the lovely 'hairy' lichen you often find in northern Scotland and the Isles. The outline of the chamber is easy to see and there are many large kerb stones on its southern edge still in situ. There is also a lovely large slab of stone which is largely made up of a pink quartz.
Although this is only my first afternoon on the island (another days adventures awaits tomorrow!) I would say this is a 'must see' site if you ever happen to be anywhere near Port Ellen.