It’s amazing to me how few people have ever heard of this story, in my opinion it should be as well known as the Iliad. The summary below, from articlesbase.com, is the most concise version of I could find.
Tain Bo Cuailnge meaning the Cattle Raid of Cooley is a legendary epic from early Irish literature. The Táin Bó Cuailnge represents the oldest vernacular tale of Western Europe, predating both Beowulf and Homer’s Odyssey. It describes the invasion of Ulster by the armies of Queen Medb of Connaught and her husband Ailill intending to steal the Brown Bull of Cuailnge. They are faced only by the boy warrior Cuchulainn, the rest of the men of Ulster being incapacitated by an ancient curse placed by the ancient Celtic goddess of war, Macha. Cuchulainn is young enough to be free of the curse, he manages to hold off the invading armies until the Ulstermen are free of the curse.
Read the rest of the summary here.
Here is a look at the version of the story I have read. It can be purchased at Amazon.com