Ken's magical photos of this site must, I assume, show the renowned Holy Mountain of Croagh Patrick in the background. Apparently there's not just a stone row here, but a stone pair, three isolated stones, a possible stone circle, some mounds and an enclosure - quite a lot going on. There's an article about it in the Journal of the Galway Archaeological and Historical Society for 1998 (v50), by Christiaan Corlett.
[A] local story suggests that the standing stones at Killadangan are "a pagan cemetery, and that the ashes of Firbolg chiefs lie in urns beneath the boulders" (Quinn's 'History of Mayo, v2', 1993). The legend appears to represent local explanations for the monuments at the site.
Perhaps the most intriguing folk-tale about the site is recorded by a local story teller, James Berry, who relates a story in which the king of Killadangan was the brother of Queen Maeve's first husband (Horgan, ed. 'Tales of the West of Ireland', 1988). The name of this "great pagan king" seems to have disappeared from local tradition, whereas the name of his lazy servant, Thulera, remains in folk memory. In this story the king makes a vain attempt to force the sea and tide under his obedience. As the king awaits the incoming tide, his servant falls asleep, and the monarch is forced to fight a single-handed battle wielding his sword against the encroaching sea. Both the king and Thulera are drowned for their efforts. This story appears to explain the encroachment of the sea into the area around the standing stone monuments.
The article also suggests there is a winter solstice alignment between the stone row and the mountain, and that the axis of some of the standing stones could also be related to the mountain.