Monsters of Celtic Mythology by Bernard Evslin

In his quest for revenge, a Celtic hero must outwit an ice-breathing dragon and an evil, shape-shifting sorceress
Yards shorter than his brothers and sisters, Celtic giant Finn McCool is the runt of the litter. Still, he is eager to fight evil and is consumed by the need to avenge his father's murder.
Thwarting his mission is Drabne of Dole, who can change shape at will. Also known as the Fish Hag and the Winter Witch, she fiercely guards her underworld terrain, keeping a watchful eye on the Salmon of Knowledge, lest he try to teach ignorant creatures what they have no right to learn. Now she is scheming to destroy Finn, who has just been given a precious gift by the Thrig of Tone, himself imprisoned in the wicked sorceress's spell. Finn will need all his courage and cunning to outwit Drabne and lay a trap for Goll McMorna, the war chief who slew his father.
On his journey from boyhood to manhood, Finn meets an ancient wizard, an ice-breathing dragon, and a fiend named Vilemurk, among others, in this imaginative retelling of one of Ireland's most enduring myths.