I visited this site last year. I parked as close as I could to the top of the hill and then started the long walk up. It was a hot day and by the time I reached the top I was knackered! Easy to find - just head for the large arial. Good views when on top.
Wikipedia. You're never quite sure if it's genuine or misinformation. But anyway, that never stops me normally, and it says:
Saint Peter was visiting Wales in order to watch over the Faithful. Taking offence at the sudden appearance of the Devil, he picked up a large number of boulders and placed them in his apron so as to carry them more easily. He then gave chase to the Devil, both chaser and chased (having the stature of giants) leaping from mountain-top to mountain-top. As the Devil alighted on Mynydd Machen he paused to catch his breath, whereupon Saint Peter began hurling the rocks at him, leaving a considerable amount of debris around his adversary in the process. The area of rocks is known to this day as "The Devil's Apron Strings".
The name of the cairn on top of the mountain, Twyn y Certhi could imply 'Cerddi'? and thus mean the mound of singing/poetry. But perhaps someone knows better.