The EU made the promises, and delivered on them, that facilitated Republican agreement to the GFA.
In a nutshell, the removed many of the triggers for conflict and gave a reason to negotiate with Britain.
That is crucial in my book.
So many various protagonists involved. Hard to actually be sure who had biggest impact.
The role of the ordinary people was key to me though. People were sick and tired of the sickening violence and wanted to live quietly in peace with their neighbours.