OOC: I already said that I will deal with Austria laterz. And tirol is a useless piece of land that is Austrian anyways.
OOC: Eh, not really useless. Trentino's majority Italian, and Alto Adige/Sudtirol has a significant Italian minority. More importantly, though, Sudtirol also commands the Brenner Pass, which makes it strategically invaluable to protect eastern Italy. That is part of the reason why Italy was given the Sudtirol originally, in the historical post-war settlements. It's big, it's (relatively) low-altitude, and it's also one of the very few passes in that area.
I can actually see irredentists accepting going against France instead of Austria-Hungary, but the terrain's actually even harder going west than going east, where you can at least punch into Dalmatia and Istria by following the
smoother coast. Austria-Hungary was also a three-time foe, but even that isn't completely impossible to justify, judging by how Austria-Hungary is presently working alongside Germany, or how Bulgaria and the Ottomans historically came together against historical Bulgarian friend Russia, and especially since you can still spin the Triple Alliance as defensive in nature (after all, Serbia shot first in Sarajevo, right?). Just don't let the war go poorly, or you'll be looking at what historically happened to Bulgaria, where the peace movement unites with the nationalists upset with your choice of allies to topple the government. Given that you're fighting through the French Alps with an army that couldn't outfight
Austria-Hungary...