*** Some people are saying the 1.3 update has fixed their Morrigan trial problem on the Series X. So definitely make sure your game gets the update. ***
If you're still unable to pop this:
The Mastery Challenge update 1.2.2 may have messed up your chance for this achievement if you haven't done all the Trials of the Morrigan - specifically the one near Lough Rea (lake) in Connacht.
There's a chance this only affects Series X players. You might be able to pop it on an Xbox One model.
To 'complete' a territory, you need to finish all mysteries and collect all wealth and artifacts. I'm not sure if viewpoints and trading posts count, so do them too, to be sure. Some things can only be done when the story lets you into certain areas. You can do this before you finish the story. I got all mine done before meeting up with King Flann again in Knockfree (Gathering Strength quest).
There are 4 main territories in Ireland. All are Level 55. There are 8 viewpoints.
(if you need help with specific things, the links are to guides provided by PowerPyx.com)
Dublin - 8
wealth, 2
mysteries, 4
artifacts Meath - 15
wealth, 4
mysteries, 8
artifacts Connacht - 18
wealth, 6
mysteries, 9
artifactsUlster - 28
wealth, 5
mysteries, 8
artifacts Complete all of those to get the achievement.
Mysteries include altars, cairns, events, side quests, Drengr fights, Trials of the Morrigan, etc.
Artifacts include hoard maps, flying tattoos, cursed symbols, Ui Neill figurines, etc.
Wealth includes Tungsten ingots, Armor, Books of Knowledge, raid chests, etc.