This is the most famous and extensive of the legends of Cúchulainn and is the story of the war fought over the Brown Bull of Cooley (Cualgne). 

This is  the story of two bulls of the Sidhe (the race of immortals).  The bulls themselves were immortal and had been transformed from other forms and origins.  

First they were the swineherds of the gods Bodb (King of the Sidhe of Munster) and Ochne (King of the Sidhe of Connaught).

The two swineherds were in rivalry with one another, changing shape in pursuit of their endless quarrel.  They became ravens and battled for a year, then they changed to water creatures, then they changed to human champions and then finally into eels.  

One of these magical eels swam into the River Cruind in Cualgne in Ulster and was swallowed by a cow belonging to Daire of Cualgne.  The other swam into the spring of Uaran Garad, in Connaught, where it was swallowed by a cow belonging to Queen Medb.

From these origins were born two bulls, the Brown Bull of Ulster, and the White Horned Bull of Connaught.

The White Horned Bull did not want to be the property of a woman, so he wandered into the herd of Ailill husband of Medb.  When they were surveying their property one day they discovered that they had equality in all possessions except that Medb had no bull to equal the White Horned Bull in her husband's herd. 

(Having equal possessions was important to a marriage for the head of the household was determined by their property and it could be either a male or female depending on who owned the most goods).

So Medb determined to get a loan of the Brown Bull of Cooley so that she might appear equal to her husband.  One version of the story has it that she "offered Daire the knowledge of her upper thighs" in other words offered to have sex with him if he would lend her the bull,  he agreed at first, and broke his cushion bouncing on it for joy, and spilling feathers out of it everywhere.  However the agreement did not last as a servant of Daire's overheard the messengers from Connaught boasting that if the bull had not been lent them they would have taken it by force, the servant reported this back to Daire and he reneged on his former agreement.  

So began the great war of the Cattle Raid with Queen Medb raising her army to take the bull by force.   She thought this would be an easy task as all the champions of Ulster were under the effects of the ancient curse of the Goddess Macha, who cursed them to feel the pains of labour at a certain time each year because of an insult she had received from an ancestor of King Conchobar's.

Medb sought a prediction from her magician Calatin, concerning the outcome of the battle, and he just told her that even if no-one else returned that she would.  As she was riding in her chariot a woman of the Sidhe appeared to her called Feidelm and told her that she saw all the men of Connaught covered in crimson blood.  Medb questions the verity of her words saying " but all the great champions of Ulster are afflicted with the curse of Macha at this time..." but Feidelm says "I see a man who performs weapon feats, the hero's light is on his brow, he is young and he resembles Cúchulainn of Muirthemne... this much I know that by him shall the host of Connaught be bloodied."

Cúchulainn because of his divine heritage was not affected by the curse of Macha and it fell to him to defend Ulster single-handed against the army of Connaught.  This was possible because of the Celtic method of single-combat - a champion would be chosen to fight against a champion from the opposing side.  This meant that there was not as much carnage as occurs in modern warfare and only those specifically trained in Warcraft would be involved.  He managed to kill any of the champions of Connaught that came against him in duels and many more from a distance with his sling.  He slew one of Medb's serving girls because she had been wearing Medb's gold headdress and he mistook her for the queen.

Eventually Medb seeing how the duels were going and wanting to meet Cúchulainn face to face sent one of her bards to ask if he would meet with her.  He agreed and she was amazed to be faced with this young man of only seventeen years without beard, she offered him her friendship and to sleep with him (one version has her going to him naked) with great honour and possessions if he would leave Ulster and come work for her, but he refused her offer.  In the end he offered her terms of battle by which as long as he was in combat with one of her champions her main army was allowed to move forward, but as soon as combat ended her army were to stop where they were.   Medb agreed to these terms as she thought it a better bargain to gain a little ground daily, then to lose many men and gain no ground at all.

Medb's champions could not defeat Cúchulainn, but they managed to engage his attention long enough so that Medb could steal the Brown Bull and fifty heifers and head back to Connaught with them.

The Brown Bull of Cooley had been captured and travelled with Medb's army into Connaught, where he challenged the white bull of King Ailill, during the terrible battle which happened between them the Brown Bull ripped the White to pieces tossing his loins as far as Athlone and the liver to Trim.  After the fight the Brown Bull returned to Cualgne, where he became maddened with rage and killed all before him, finally his heart burst with the pressure of all this exertion and so he died.  This ended the cattle raid of Cooley.

Further Reading:

J. Dunn - Táin Bó Cualgne 1914. translation.

Thomas Kinsella - The Táin, translation. Oxford University Press, 1970.

Liam Mac Uistin - The Táin, The Great Celtic Epic. The O'Brien Press. Dublin 1989.

Cecile O'Rahilly editor - Táin Bó Cuailgne from the Book of Leinster. (text and translation)  Dublin Institute for Advanced Studies.  1946.