Conversation so far:
When did the reign begin?
The kingdom of Gwynedd began with the reconquest of the coast by northern Britons.
Who were the Britons fighting?
Gaelic tribes from Ireland.
How did the conquest go?
Generations later, Maelgwn's father Cadwallon Long-Hand completed the process by destroying the last Irish settlements on Anglesey.

Question: Who started the conquest?
Self-contained rewrite: Who started the conquest of the Gaelic tribes from Ireland?