Amairgin was the son of Cas mac Baicc meic Rosa Rúaid meic Rudraige. He overtook the hosts going westwards over Tailtiu and he turned them and drove them northwards over Tailtiu. He lay on his left elbow in Tailtiu and his people furnished him with stones and rocks and great flagstones and he fell to pelting the men of Ireland for three days and three nights.