One famous short Celtic story is 'The Salmon of Knowledge'. In this story, Fionn Mac Cumhaill sought the wisdom held by the Salmon of Knowledge. After catching and cooking it, he burned his thumb while testing if it was cooked. He put his thumb in his mouth and thus gained the fish's knowledge.
The story of 'Cú Chulainn's Boyhood Deeds' is quite interesting too. As a boy, Cú Chulainn showed extraordinary strength and courage. He defeated the fierce hound of Culann and then took its place, protecting Culann's property. His feats from a young age were a sign of the great hero he would become.
There's also the story of 'The Children of Lir'. Four children were turned into swans by their jealous stepmother for 900 years. They endured great hardships and long years in different forms, and it's a tale full of magic, transformation, and the power of love in the face of adversity.