I would venture a guess as to what that means to us today. A primitive, earthy, pagan, nationalism that seeks to cast off civilization (Christianity) and seek to live like the phony-baloney "noble savage" imagined by Rousseau.
Am I at least close?
Like all crazy human ideas that mutate in all directions, yes, sometimes. The 19th century thing was much more highbrow intellectual though.
`Italia irredenta’ referred IIRC to regions in Italy’s far north that were claimed/ruled by other nations despite their ethnic Italian majority. Trieste and South Tyrol come to mind (Austria, then Yugoslavia) where the place name signs still display Italian/German versions. This was expanded by the irredentists to include Corsica, Nice, and even the Greek island of Corfu.
Irredentism provided Hitler the perfect excuse for annexing the Sudetenland given its majority German population. The problem with irredentism is that one never knows when to stop.