As I suspected, your error is in the ireland section, starting at line 3468 (see below). There are also a few other weird entries. Settlements with about 60 buildings, most of them triple. Factions without settlements. This might have been caused by not verifying changes or maybe simply forgetting to move the old garison (less likely).
My suggestion: get your backup copy and start again. After that change the ownership of the settlement - verify. Move the (now invalid) garison inside the settlement somewhere else - verify. Create a new (valid faction) garison - verify. Save. Follow my suggestion regarding the historic_event file. Try if it works.
Code:
character Brian O'Neill, named character, male, leader, age 39, x 9, y 230
traits Factionleader 1 , MilitaryInclination 1 , NaturalMilitarySkill 1 , GoodCommander 1 , BattleDread 3 , StrategyDread 1 , NightBattleCapable 1 , Intelligent 1 , ReligionStarter 1
army
unit NE Bodyguard exp 1 armour 0 weapon_lvl 0
unit Spear Militia exp 1 armour 0 weapon_lvl 0
unit Deisi Javelinmen exp 1 armour 0 weapon_lvl 0
unit Deisi Javelinmen exp 0 armour 0 weapon_lvl 0
unit Peasants exp 0 armour 0 weapon_lvl 0
unit Hobiguir exp 0 armour 0 weapon_lvl 0
unit Hobiguir exp 0 armour 0 weapon_lvl 0
unit Hobiguir exp 0 armour 0 weapon_lvl 0
unit exp 0 armour 0 weapon_lvl 0
unit exp 0 armour 0 weapon_lvl 0
unit exp 0 armour 0 weapon_lvl 0
unit exp 0 armour 0 weapon_lvl 0
character Domnall O'Neill, named character, male, heir, age 20, x 13, y 218
traits Factionheir 1 , MilitaryInclination 1 , Intelligent 2, PublicFaith 1 , LoyaltyStarter 1, ReligionStarter 1
army
unit NE Bodyguard exp 0 armour 0 weapon_lvl 0
unit Spear Militia exp 1 armour 0 weapon_lvl 0
unit Spear Militia exp 1 armour 0 weapon_lvl 0
unit Hobiguir exp 0 armour 0 weapon_lvl 0
unit Hobiguir exp 0 armour 0 weapon_lvl 0
unit Hobiguir exp 0 armour 0 weapon_lvl 0
unit exp 0 armour 0 weapon_lvl 0
unit exp 0 armour 0 weapon_lvl 0
unit exp 0 armour 0 weapon_lvl 0
unit exp 0 armour 0 weapon_lvl 0