The following is the Argument of the Tain Bo Cualnge, which, for the sake of convenience, is here divided into sections:
One night at the palace of Cruachan in Connacht, a dispute arose between Queen Medb, the sometime wife of Conchobar, king of Ulster, and her consort Ailill, as to the amount of their respective possessions. It may be remarked in passing that in those days in Ireland, married women retained their private fortune independent of their husbands, as well as the dowry secured to them in marriage. To procure the evidence of their wealth, the royal pair sent messengers to assemble all their chattels which, on comparison, were found to be equal, excepting only that among Ailill’s kine was a lordly bull called Finnbennach, “the Whitehorned,” whose match was not to be found in the herds of the queen.
As we might expect, Medb was chagrined at the discovery. Now her herald macRoth had told her that Dare macFiachna, a landowner of Cualnge, a district in the territory of her former husband, possessed an even more wonderful bull than Ailill’s, called Donn Cualnge, “the Brown Bull of Cualnge.” So she despatched macRoth to Dare to pray for the loan of the bull.
Dare received the queen’s messengers hospitably and readily granted her request, but in the course of the entertainment, one of the messengers, deep in his cups, spoke against Dare, and he, hearing this, withdrew his promise and swore that he would never hand over the Brown Bull of Cualnge.
The impetuous queen, enraged at the failure of her mission, immediately mustered a formidable army, composed not only of her Connachtmen but also of allies from all parts of Ireland, wherewith to undertake the invasion of Ulster. On her side were the Ulster chieftains who had gone into exile into Connacht after the treacherous slaughter of the sons of Usnech by King Conchobar of Ulster. Chief among them was Fergus, who, moreover, had a personal grievance against Conchobar. For, while Fergus was king of Ulster, he had courted the widow Ness and, in order to win her, promised to abdicate for the term of one year in favour of her son Conchobar. But when the term had elapsed, the youth refused to relinquish the throne, and Fergus in anger entered the service of Medb of Connacht. There he was loaded with favours, became the counsellor of the realm and, as appears from more than one allusion in the tale, the more than friend of the wife of King Ailill.
The four leagued provinces of Ireland being gathered at Cruachan, the guidance of the host was entrusted to Fergus, because he was acquainted with the province of Ulster through which they were to march, and at the beginning of winter—a point emphasized by the exponents of the sun-theory—the mighty host, including in its ranks the king and queen and some of the greatest warriors of Ireland, with the princess Finnabair as a lure, set forth on the raid into Ulster.