The Boyhood Deeds of Cu Chulaind
Celtic heroes were mostly a precocious bunch and Cu Chulaind is a prime example, able to beat the other boys (all three fifties of them (at once)) at any feat of prowess and kill with his bare hands a dog that all the adult warriors are scared of. He goes on to single-handly defend Ulster from the assembled forces of most of the rest of Ireland for days.
The Cattle Raid of Froech
Cattle raiding is a Big Thing in the Irish Celtic stories. It also was a historical fact of the Scottish Highlanders' way of life. I'm not sure if it was pan-Celtic or not. Anyway, Froech wants to marry the daughter of Mebh but he's opposed at every turn and adventures ensue. Lots of Celtic literature motifs, including, rings, fairies, salmon, lake monsters, cattle raiding, feasting and complicated tasks required of the hero in order to get the girl...