CLOCH-A'-PHOILL, (literally the hole stone, in Irish.)
-- Two miles south of Tullow, in the parish of Aghade, is a huge piece of granite of singular appearance. It is about twelve feet in height and four in breadth, having an aperture through near the top.
There is a tradition, that a son of one of the Irish kings was chained to this stone; but that he contrived to break his chain and excape. There are marks left, caused by the friction of the iron on the stone. We would at once conclude that it was a bull, or some other animal that was chained here, and not a human being; were not the tradition confirmed by written history, the verity of which we are not disposed to controvert.*
The stone is now thrown from its perpendicular, and it was a practice with the peasantry to pass ill-thriven infants through the aperture in order to improve their constitution. Great numbers formerly indulged in this superstitious folly, but for the last twenty years the practice has been discontinued. My informant on this occasion was a woman who had herself passed one of her infants through the aperture of this singular stone. She informed me, that some of the country people talked of having it cut up for gate posts, but a superstitious feeling prevented them. Every antiquary would regret the demolition of the cloch-a-phoill.
Elsewhere in the book (p19) he describes the story. I will try to summarise it because it's pretty wordy. He doesn't seem to notice the irony when he says "We shall relate it with as much brevity as may be consistent with a due regard to perspicuity." But to be fair it is complicated.
Niall was the rightful king. But Eochaidh sets himself up at Tara as the king instead. 'A druid of eminence' tells him off and he scarpers. Soon after Eochaidh kills yet another druid for some ill-timed comments(I think). Niall promises the family that there'll be revenge. But he ends up trashing Leinster in his pursuit of Eochaidh. The people of Leinster end up handing him over to prevent any more trouble. The druid chains him to the stone, and then gets nine soldiers to attack him. But Eochaidh manages to make a superhuman effort and forces one of the chain's rivets. He grabs some weapons, hacks down the soldiers and dashes off to Scotland..
p338 in 'The History and Antiquities of the County of Carlow ' by John Ryan, 1833. Digitised on Google Books.