I would argue that Italy made a serious strategic mistake in the Great War by bashing her head against the Izonzo. Her forces would have been much more decisive deployed into other theaters, especially (IMO) the Balkans. Indeed, her alliance with Germany envisioned deploying two army corps to Lorraine, where the war was likely to be decided. Italy's peacetime army was around 250,000; I think that it's plausible that Italy could put a few divisions into Gallipoli.
The bigger question is whether earlier Italian entry and involvement against the Ottomans (and possibly in support of the Serbs or Montenegro) would have an immediately decisive impact, or merely shift the balance of forces somewhat further in the Entente's favor.
My thoughts- While OTL's front was the most obvious way to the irredenta, the authors here have a point about not taking advantage of other theaters while underestimating terrain challenges on the common land border with Austria.
One possibility is always to throw substantial Italian troops into the French front, perhaps even Alsace, just on the opposite side from those old plans contemplated with Germany. Of course that would have to be accompanied by an early political decision in 1915 to declare war on Germany as well Austria-Hungary, because in OTL, Italy hesitated to declare war on Germany directly until some point in 1916.
Other options for the Italians include landings on the Dalmatian coast. I am not sure about the prospects of such operations to land and secure ground, and then from there, to exploit inland. Other activities the Italians might usefully pursue in 1915 might be to defensively reinforce the Montenegrins, and get much needed supplies through to the miraculously surviving Serbs. Logistical road conditions aren't great, but with some early timed support for the Montenegrins, the fall of Montenegro to Austrian occupation can probably be avoided indefinitely, or at least the fall of most of it, and it can remain a perpetual thorn in Austria's side. Supply to Serbia could possibly delay or prevent its fall. If the Montenegrin foothold can be built up over the months to logistically support wider employment of Italian forces, despite the local mountains there still may be more room for mobility and initiative than on the Isonzo.
Other Italian options include against Ottoman targets, north of Gallipolli, or at its opposite Asian shore, which can double as demonstrating the value of having the Dodecanese in Italian hands. Any Italian pecking at Ottoman peripheries, alone, or in tandem with the British in the Levant or around the Arabian peninsula, can be helpful to the larger Entente cause to speeding Ottoman collapse and the relief that would provide to Russia.