=== 青年時代 ===
==== カスバドの予言 ====
ある日のこと、クー・フーリンはドルイドのカスバドが、今日騎士になるものはエリンに長く伝えられる英雄となるが、その生涯は短いものとなるという予言をしたのを聞き、騎士となるべく王の元へと向かった。騎士になるにはまだ早いと渋る王に対して、クー・フーリンは槍をへし折り、剣をへし曲げ、チャリオットを踏み壊して自身の力を見せつける。観念した王はクー・フーリンが騎士になるのを許し、彼の力にも耐えられる武器とチャリオットを与えた。
One day at Emain Macha, Cú Chulainn overhears Cathbad teaching his pupils. One asks him what that day is auspicious for, and Cathbad replies that any warrior who takes arms that day will have everlasting fame. Cú Chulainn, though only seven years old, goes to Conchobar and asks for arms. None of the weapons given to him withstand his strength, until Conchobar gives him his own weapons. But when Cathbad sees this he grieves, because he had not finished his prophecy—the warrior who took arms that day would be famous, but his life would be short. Soon afterwards, in response to a similar prophecy by Cathbad, Cú Chulainn demands a chariot from Conchobar, and only the king's own chariot withstands him. He sets off on a foray and kills the three sons of [[Nechtan Scéne]], who had boasted they had killed more Ulstermen than there were Ulstermen still living. He returns to Emain Macha in his battle frenzy, and the Ulstermen are afraid he will slaughter them all. Conchobar's wife [[Mugain]] leads out the women of Emain, and they bare their breasts to him. He averts his eyes, and the Ulstermen wrestle him into a barrel of cold water, which explodes from the heat of his body. They put him in a second barrel, which boils, and a third, which warms to a pleasant temperature.{{sfn|Kinsella|1969|pages=84–92}}
==== その他 ====
クー・フーリンは、フォルガル(Forgall Monach)の娘エメルに求婚するが断られたため、影の国を訪れ女武芸者スカアハの下で修行を行う。この時共に修行を行った仲間に、コノートのフェルディアがおり、彼とクー・フーリンは親友、そして義兄弟となる。