Republicanism is an ideal formed in Ancient Rome, with the Senate electing Consuls and there being tribunals, at it's core, a republic is a representative democracy with themes of nationalism.
The Most Serene Republic of Venice, with it's elected Doges, is a prominent European republic of this time seen in Renaissance Kingdom as a powerful Italian nation far to the North of realms like the Kingdom of Two Sicilies.
Republics in ideaology have been imagined by some criminal, anarchist, or revolutionary forces which have often failed in the Kingdom of England, Kingdom of Ireland, or Kingdom of Scotland. Some of these ideals, at least in form, have been persisted as a theme to some forces such as The Populares Party which once existed, and in affiliation, the Wolves of Sherwood, a criminal organization. Even organizations like NNGO and Fury would ensue anarchy and sectionalism and decentralized governments through their campaigns in nations against forces of authority such as the King of England or King of Scots. Republicanism is thus a respected institution in some places, but a theme for revolutionary cause in others where it is not institutionalized.