Our knowledge of Irish druidism is derived chiefly from incidental references in the old romantic tales, and also in the _Lives of the Saints_, and especially in the _Lives of St. Patrick_, who came into direct antagonism with their entire system. It is certain that in other countries the Druids sometimes offered human victims in sacrifice; and there is some evidence that the same custom, although, perhaps, more rarely, prevailed in Ireland. There is a passage in the _Book of Leinster_,[7] which expressly states that the Irish used to sacrifice their children to Crom Cruach, or more correctly, Cenn Cruaich, the great gold-covered idol of Magh Slecht, on the borders of Cavan and Leitrim. Hence it was called the Plain of Slaughter, and the sight of the foul idol so excited the righteous zeal of St. Patrick that he smote it deep into the earth with a blow of his crozier. We also know from the Saint's "Confession" that the Irish to whom he preached the Gospel, had previously worshipped idols and unclean things,[8] which goes to prove that idol-worship was a part of the druidical ritual in Ireland. There is no doubt also that the worship of the elements was a part of the druidical religion. Their most terrible oaths were sworn on the Sun and the Wind; and it was confidently believed that the perjurer could never escape the vengeance of these mighty elements. The account given in the _Tripartite_ of St. Patrick's interview with the daughters of King Laeghaire by Cliabach Well, on the slopes of Cruachan, shows that the worship of fairy gods, or elves, was a part of the druidical religion; and the same is expressly stated in the very ancient metrical Life of the Apostle, by St. Fiacc of Sletty.[9]