I found some wonderful folklore on duchas.ie about the tomb, also known as Darby's Bed. Much of the action in this story takes place in Co. Tipperary and initially when I saw sunset mentioned, I thought that it might have had some hidden astronomical meaning. However (as above) the passage in Darby's Bed is aligned to the north. Interestingly there is a wedge tomb at Corderry that is almost exactly 3 miles away and a passage tomb at Shrough that is almost 4 miles away.
"A long, long time ago there lived a fierce black pig on the Sliab na Muc Hills. This pig was the terror of the neighbourhood and nobody would dream of roaming on the hilltops for fear of meeting with this terrible beast. It happened that one day a great giant called Diarmuid chanced to come across the pig at a point or peak on those hills called Corrin (Cairn). The pig in its rage started rooting up the ground and continued doing so until it had made a huge deep hole. This hole is still to be seen and for years everybody that passed it by threw a stone into it, why I cannot say.
The giant then attacked the pig put could not kill it. The battle raged for hours and towards sunset they found themselves 3 or 4 miles further east at a place called Rathdarby. The giant here made a last attempt with his spear to kill the pig but he missed his
thrust only to find the pig taking a sudden hold of him on the ground. It then seized the giant by the throat and killed him. He was buried on the spot where today three large stones mark the site of his grave. This grave is on Mr. T. Kennedys land Rathdarby, a few miles from here.
All the old people called the valley lying south of the Sliab na Muc Hills the "Valley of the Black Pig"."
The giant then attacked the pig put could not kill it. The battle raged for hours and towards sunset they found themselves 3 or 4 miles further east at a place called Rathdarby. The giant here made a last attempt with his spear to kill the pig but he missed his
thrust only to find the pig taking a sudden hold of him on the ground. It then seized the giant by the throat and killed him. He was buried on the spot where today three large stones mark the site of his grave. This grave is on Mr. T. Kennedys land Rathdarby, a few miles from here.
All the old people called the valley lying south of the Sliab na Muc Hills the "Valley of the Black Pig"."