As far as I am aware, Patrick's life has been reconstructed by historians from a limited number of events or locations where he was believed to be present.

The links between these as "established" by historians have been a matter of conjecture and several views exist.

Lawhead seems to have taken the "facts", the elements of interpretation upon which most people are agreed and some of the other conjectures and built a story. Neither more nor less factually correct than any others, but a cracking read nonetheless!