The mare sacrifice story comes from Giraldus Cambrensis, Topography of Ireland, a 12th Century source.
Here is a translation:
Chapter XXV Of a new and monstrous way of inaugurating their kings.
There are some things which shame would prevent my relating, unless the course of my subject required it. For a filthy story seems to reflect a stain on the author, although it may display his skill. But the severity of history does not allow us either to sacrifice truth or affect modesty; and what is shameful in itself may be related by pure lips in decent words. There is, then, in the northern and most remote part of Ulster, namely, at Kenel Cunil, a nation which practises a most barbarous and abominable rite in creating their king. The whole people of that country being gathered in one place, a white mare is led into the midst of them, and he who is to be inaugurated, not as a prince but as a brute, not as a king but as an outlaw, comes before the people on all fours, confessing himself a beast with no less impudence than imprudence. The mare being immediately killed, and cut in pieces and boiled, a bath is prepared for him from the broth. Sitting in this, he eats of the flesh which is brought to him, the people standing round and partaking of it also. He is also required to drink of the broth in which he is bathed, not drawing it in any vessel, nor even in his hand, but lapping it with his mouth. These unrighteous rites being duly accomplished, his royal authority and dominion are ratified.
http://www.archive.org/details/historic ... 00girauoft
As you can see, Giraldus is not unbiased; and since he retails some very tall stories elsewhere in the book, he cannot be regarded as particularly reliable either; Irish historians have frequently raged against him. Bu there it is, for what it is worth!