When he was old enough the boy Setanta (Cu Cuchulain) went to the court of Conor (King of Ulster) to be brought up and instructed along with the other sons of princes and chieftains. It was now that the event occurred from which he got the name of Cuchulain, by which he was hereafter to be known.

One afternoon King Conor and his nobles were going to a feast to which they were bidden at the dun (keep) of a wealthy smith named Cullan, in Quelgny, where they also meant to spend the night. Setanta was to accompany them, but as the cavalcade set off he was in the midst of a game of hurley with his companions and bade the king go forward, saying he would follow later when his play was done. The royal company arrived at their destination as night began to fall. Cullan received them hospitably, and in the great hall they made merry over meat and wine while the lord of the house barred the gates of his fortress and let loose outside a huge and ferocious dog which every night guarded the lonely mansion, and under whose protection, it was said, Cullan feared nothing less than the onset of an army.

But they had forgotten Setanta! In the middle of the laughter and music of the feast a terrible sound was heard which brought every man to his feet in an instant. It was the tremendous baying of the Hound of Cullan, giving tongue as it saw a stranger approach. Soon the noise changed to the howls of a fierce combat, but, on rushing to the gates, they saw in the glare of the lanterns a young boy and the hound lying dead at his feet. When it flew at him he seized it by the throat and dashed its life out against the side-posts of the gate. The warriors bore in the lad with rejoicing and wonder, but soon the triumph ceased, for there stood their host, silent and sorrowful over the body of his faithful friend, who had died for the safety of his house and would never guard it more.

"Give me," then said the lad Setanta, "a whelp of that hound, O Cullan, and I will train him to be all to you that his sire was. And until then give me shield and spear and I myself will guard your house; never hound guarded it better than I will."

And all the company shouted applause at the generous pledge, and on the spot, as a commemoration of his first deed of valor, they named the lad Cuchulain, the Hound of Cullan, and by that name he was known until he died.

The previous text was quoted from the book:
Title: Celtic Myths and Legends
Author: T.W. Rolleston
Publisher: The Gresham Publishing Company (1994)