Walked here from Carreg Maen Taro 14.8.2009. I had intended a trip over to Blorenge cairn, but by the time I got to the WT station at Cefn y Galchen, the drizzle had become persistent and I headed straight here.
There are two cairns mentioned on Coflein. According to the OS, the named "Carn-y-Defaid" is the SW of the two. One prominent cairn is immediately visible from the WT station, being situated on the horizon. This makes navigation easy, even in the rain.
As I got nearer, it was apparent that the visible cairn is on the NE side of the path, which would not make it the named cairn but the other one. It's a great big monument, an impressive mound of stones even though the centre has obviously been dug into. The drizzle didn't encourage me to linger, especially as the exposed situation also added a pretty stiff wind. So, hood up and visibility restricted, I crossed the path the look for the SW cairn. I found two small-ish "cairns" both obviously much tampered with. I wouldn't like to commit to either of these being the named Carn. Maybe I missed another larger cairn, but really didn't fancy lingering any longer.
I headed back to Blaenavon along the footpath across Mynydd y Garn fawr, passing a couple of upright (natural?) stones on the left hand side.
[Edit 20.2.2010: Seen from Carn Blorenge on a much nicer day, it looks as though the large cairn is Carn-y-Defaid. There is another large cairn just down the slope to the NE, which was invisible to me in the mist when I came up before. A return trip to check this out is on order.]