Head out on the road from Port Ellen to Ardbeg. After Lagavulin you will come to the junction with the road to Ardbeg on the right. If you turn left instead there is a small road heading up to the farm at Callumkill. As you go up this road, this cairn is visible on the hill to your right. Walking up to it is a bit of a trek through the bracken, best to check with the farm too because sometimes there's a bull in the field next to it.
No idea of its age - doubtless people have built/rebuilt/tidied it up over the years. I first visited it about 35 years ago as a small child, when my grandad worked on the farm as a shepherd. I remember putting stones on it myself (it already existed at this stage of course). On the other hand there were non-naturally occurring stones lying around the cairn, so even if it was (re?)built relatively recently people may have used materials left over from a more ancient monument.