This is the Tain, or Tain Bo Cuilgne, or the "Cattle Raid of Cooley" as retold in verse by the Decemberists.
What's going on? Well, the King and Queen of Connaught get into an argument about who has more wealth. It turns out the King is very slightly ahead by owning a prize bull. So the Queen calls out the army to invade Ulster and steal the Brown Bull of Cooley. The army is repelled by the hero CuChulainn, who is the only warrior of Ulster available to fight due to a curse.
CuChulainn is nigh invincible when he is in his battle rage, which transforms him into a monster. He lays waste to the Connaught army. But eventually the Queen sends the warrior Ferdiad, a boyhood friend of CuChulainn, to face the hero.
CuChulainn survives, and chases the Queen and the army out of Ulster, but they take the Bull of Cooley with them. The two prize bulls meet in a pasture and fight. The Bull of Cooley impales the other bull, then breaks through the fence and runs home to Cooley, where its heart explodes.
Yes, like so many wars, this one was a waste of time and life. And this story dates from the pre-Christian era of Ireland; some truths are eternal. If the outline of the story doesn't seem like much, pay close attention to the video, which relates the story with more poetry -- and shadow puppets!