Wolf's Quest by Don Baldwin

A feral child named Wolf is being raised by wolves deep in the forest, until he is suddenly whisked away to a subterranean netherworld by the Tuatha De Danann; a strange and elusive race. In this otherworldly realm, where mortal time moves imperceptibly quickly, these 'ever-living ones' raise and train the boy for their own ends; returning a formidable stealth-warrior, back to the world of men. In Wolf's absence, Ireland has become a more dangerous place, as the Vikings strive to gain control of the country. However, a young southern commander called Brian Boru and his enigmatic counsellor, Amergen, mean to thwart the Viking' plans. But they desperately need Wolf's particular set of skills for a perilous mission deep into enemy territory. And so they set a trap to ensnare Wolf and his daring band of Rangers.