There are 5 Cairn Dungeons in total. 2 of which can be entered at any time, the other 3 can only be accessed via quests. I've listed all 5 Dungeons below with there location or required quest:Dale Family Tomb:
Hogwarts Valley > Brocburrow. Head east from the Brocburrow Floo Flame.Tomb of Treachery:
Poidsear Coast > Tomb of Treachery. Under the bridge at the very North of Poidsear Coast.The Collector’s Cave:
Entered during the main story quest “The Helm of Urtkot.”Feldcroft Catacomb:
Entered during Sebastian Sallow’s relationship quest “In the Shadow of Time.” Moonstone Garden:
Entered during Poppy Sweeting’s relationship quest “The Centaur and the Stone.”
Note: Each dungeon needs to be entered to progress the achievement. Simply discovering the location isn't enough.