Your story is even better than the possible originals:
a) There was a clan war and the chieftan was killed. A chief could not be lame so the widow broke her infant son's leg and allowed it to set badly. This removed him as a future threat to the enemy so they allowed him to live. He became a priest and (the norm for Celtic priests) married. His descendants became an offshoot of the original clan.
b)The widow was captured while pregnant and her captors lamed the infant.
c)He was born deformed.
d)Any or none of the above minus the married part.

Hmmm, on reflection your story is much better!