It sounds like you're referencing this infamous section from Giraldus Cambrensis' Topographia Hibernia:

“There are some things which, if the exigencies of my account did not demand it, shame would discountenance their being described. But the austere discipline of history spares neither truth nor modesty. There is in the northern and farther part of Ulster, namely Kenelcunill [Tyrconnell], a certain people which is accustomed to consecrate its king with a rite altogether outlandish and abominable. When the whole people of that land has been gathered together in one place, a white mare is brought forward into the middle of the assembly. He who is to be inaugurated, not as chief, but as a beast, not as a king, but as an outlaw, embraces the animal before all, professing himself to be a beast also. The mare is then killed immediately, cut up in pieces, and boiled in water. A bath is prepared for the man afterwards in the same water. He sits in the bath surrounded by all his people, and all, he and they, eat of the meat of the mare which is brought to them. He quaffs and drinks of the broth in which he is bathed, not in any cup, or using his hand, but just dipping his mouth into it round about him. When this unrighteous rite has been carried out, his kingship and dominion has been conferred…”

Cambrensis' description of Ireland is pretty infamous for its generally dubious depiction of the indigenous Irish (except for his appreciation for Irish music). Coming to Ireland shortly after the Norman invasion of Ireland, Giraldus is often at pains to paint the indigenous Irish population as barbarians in contrast to his Norman compatriots. This has led many scholars and non-scholars to outright reject most of his seemingly derogatory characterizations of Irish culture in the 12th century, however there may be some truth behind this seemingly ghastly rite, which may have actually have been practiced along the lines of how Cambrensis described it.

What you have to understand is that Irish kingship from the early medieval period backwards could be characterized as a sort of sacral kingship. Irish kings probably originated as some sort of cultic priestly figure who was entrusted with ensuring the fertility of the territory over which he ruled, which seems to be reflected in later Irish associations of good kingship with bountiful crops and cattle, abundant game and forage, good weather etc., while bad kingship was associated with famine, disease and destruction. The defining symbolic feature of early Irish kingship was not the wearing of crowns or being seated on a throne, but the abidance of supernatural taboos and prerogatives that feature prominently in contemporary literature and legal texts. If broken, these taboos were believed to bring destruction upon the king and end his rule as seen in the literary text Togdail Bruidne Da Derga, where the Irish king Conaire Mor is forced to break each of his supernatural taboos, leading to his inevitable destruction.

Horse sex aside, Irish coronation ceremonies were nearly pre-Christian in nature by virtue of the Irish king's role as a sacerdotal figure meant to ensure his territory's fertility and uphold its metaphysical balance. Irish kings supposedly symbolically married the personification of the territory over which they ruled, and coronations often took place on ancient barrow-mounds or earthworks that had originally been constructed in the Bronze Age or even the Neolithic. In the context of the above quote, the white mare may have actually stood in to represent the fertility figure associated with territory ruled by the King of Tyrconnel.

One explanation for these seemingly archaic coronation rituals and features of the Irish office of kingship has been advanced by several scholars - that the Irish office of sacral kingship preserved a feature of Proto-Indo-European religious belief well into the early medieval period. The rite described by Cambrensis above appears to actually echo a similar rite performed by kings in India as far back as the 2nd Century BCE: the Ashvamedha. This ritual was performed by Indian kings as the most important manifestation of rulership, as it was the most important rite in the hierarchy of sacrifices. The general features of the Ashvamedha line up pretty closely with Cambrensis' description of that Irish coronation ceremony: both involved the union with the horse, its subsequent killing, the king being bathed and the horse's consumption by the king's people. Besides this sole source, it's also possible the hagiography of St. Moling makes reference to the horse-sacrifice and broth which the saint subverts by transforming it into mutton, inverting the pre-Christian associations carried by the horse's flesh and its broth.

So if you accept that Irish kingship was an archaic religious & political office that preserved some aspects of Proto-Indo-European belief which were also seen in Iron Age India, it's totally conceivable that some Irish kings might have gotten it on with a horse before having it been killed and eaten.