What is the default True King of Munster game rule in CK3?

Classic — Murchad: The Petty Kingdom of Munster is ruled by King Murchad in 1066, CK's default tutorial character since 2012. (patch 1.20.1.2)

Does changing the True King of Munster rule disable achievements?

No: none of its 2 settings carries the flag blocks_achievements in patch 1.20.1.2.

Game rule · Historicity · Flavor

True King of Munster

Classic — Murchad · default
Historic — Toirdelbach

True King of Munster is a game rule in Crusader Kings III (Historicity and Flavor) with 2 settings: Classic — Murchad (the default) and Historic — Toirdelbach. None of its settings disables achievements. Patch 1.20.1.2, from game/common/game_rules/00_game_rules.txt.

This section as JSON: /raw/game-rules.json

Quick answers (2)

What is the default True King of Munster game rule in CK3?: Classic — Murchad: The Petty Kingdom of Munster is ruled by King Murchad in 1066, CK's default tutorial character since 2012. (patch 1.20.1.2)
Does changing the True King of Munster rule disable achievements?: No: none of its 2 settings carries the flag blocks_achievements in patch 1.20.1.2.

2 settings

Classic — Murchad
Default
The Petty Kingdom of Munster is ruled by King Murchad in 1066, CK's default tutorial character since 2012.
historicity_ruler_of_munster_classic_murchad
Historic — Toirdelbach
The Petty Kingdom of Munster is ruled by King Toirdelbach in 1066, its historical ruler. Selecting Murchad as the player character will result in playing Toirdelbach.
The script checks it once, in 1 place
On actions (the game's own hooks) 1
- on_game_start_after_lobby
Every has_game_rule = historicity_ruler_of_munster_historical_toirdelbach in game/common and game/events, by the top-level object it sits in.
historicity_ruler_of_munster_historical_toirdelbach

Source: game/common/game_rules/00_game_rules.txt line 1259 (key historicity_ruler_of_munster), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.