One month! Kudos to _frms for finding you could use England to bypass war entirely (the "remote realm" calculation for the ai is a bit wonky and I had thought all of Ireland was out of range of England initially).

This will be hard to beat I think, there is not much room to reduce the time further and this run was fairly lucky - getting some perks in martial lifestyle allowed for "promising prospects" which allowed a marriage with Matilda and then the wedding even triggered very fast. I still don't know how to do this split consistently, it may be down to RNG.

Main improvement is just to start Catholic with virtuous traits and ask the pope for money, with virtues he will likely say yes. Some luck with Irish ruler traits will mean they still accept vassalisation.