long time ago in Ireland, there lived a king named Eochy. He was quite a magician, with a lot of power. He could control the weather and the seasons and the growth of the crops. You can see why he was king and why nobody would want to get on his bad side. And he was a pretty good guy too, but every now and then he would do someone a bad turn when it suited him. Anyway, it seems he got a hard-on for this beautiful woman named Eithne who lived in the rich lands near the river Boyne. She kinda liked him too (hey, he was a rich powerful king and apparently pretty buffed... for a wizard, that is). Trouble was, she had a husband... a poor dumb bastard named Elcmar.
Well, Elcmar was a land-owning chieftain under Eochy's influence, so he got sent off on some bogus diplomatic mission or other—checking on the price of grapes in Norway or some jerkoff thing. Then, while Elcmar was away, Eochy cast a long distance spell on him, making him lose total track of time; he couldn't recognize the difference between night and day or see the position of the sun in the sky. Eochy also removed his appetite so he would only nibble here and there, never having a real meal and not being able to tell the passage of time that way either. The upshot of all this hexing was that Elcmar was gone for almost a whole year, but it only seemed to him to be a few hours. While Elcmar is gone, Eochy and Eithne get together and romp around quite a bit. She gets knocked up and produces their son, Aengus.
Now, fosterage was the way in the old Celtic societies. The children of nobles were sent to live and grow up in the houses of others, while their kids were fostered somewhere else (that whole King-Arthur-growing-up-with-Sir-Kay thing). It helped form later alliances and gave the kids a little bit wider view of the world... which was a useful thing if they were going to become rulers themselves. So Eochy sees to the fostering of Aengus with another king named Midhir. Elcmar comes back and doesn't suspect a thing. Nobody told him neither... didn't want to piss off Eochy, probably.
Well, as Aengus grew up, he amazed everyone with his natural abilities as a leader. He soon became the great favourite of Midhir and everyone else too. He fell naturally into the position of Numbah One amongst all the kids fostered there with Midhir, even those much older. They all looked to Aengus before they made a move.
Of course, this did breed a bit of jealousy and one day one of the older kids teased him about being a bastard with probably no noble parentage at all (a pretty important thing back then). So little Aengus goes running to Midhir with tears in his eyes, asking if there's any truth to this. Midhir dries Aengus's tears and tells him the truth of who his father is. Well, nothing for it now but Aengus wants to meet his real Da. You see, these here old heroic characters in these old heroic stories always rush to meet their destiny like this. And they always end up getting some help along the way (read your Joseph Campbell if you don't believe me).
Midhir takes him to meet Eochy.