Irkutsk Museum of Decembrists is a focused museum in Irkutsk, the sort of place that gives useful context rather than just filling an hour. Its appeal is strongest if you want the local story behind the streets: regional history, specialist collections, or the people and institutions that shaped the city.
It works best as a deliberate stop in a things-to-do day, especially when the weather pushes you indoors or when you want a quieter counterpoint to the main squares and viewpoints. Check the official source before going, since smaller museums often keep limited opening days or seasonal hours.