Curious to see how you'll twist his story given his OTL service in the Union Army!
Ah, but what better place for a spy to be
IIRC, its peacetime organization was intended to be established on the lines of the antebellum U.S. Army, primarily manning the forts and heavy artillery along the coast and internal waterways. Technologically, it was quite Anglophilic, as the history of the Ordnance Department illustrates. Its greatest expansion, considering the Texas-Arizona question in Indians, bandits, railroads, and settlement, will likely be in cavalry, where there are many worthy officers to promote.
That was what the view was, and indeed what the Fire Eaters are interested in. But with its vast territory and security problems, that will certainly need to be revisited. The cavalry and artillery though will certainly be the purview of the Federal Government in Richmond.
If the democrats' involvement in the Fenian movement becomes common knowledge while the Fenians succeed in nothing more than scuppering any potential thaw in relations between Britain and the US right when the one thing the States' need is reliable trade partners, I can see the democratic party not surviving McClellan's presidency. Yet, if he tries to restore order while the fighting is already ongoing and the Fenians have the sympathy of a majority of the US population, it might actually go even worse for the democats.
The Fenians are going to play no small role in making life difficult for the Democrats come 1868. Truth be told, both parties are in a world of hurt when it comes to unity. The Republicans split in 1864 and both sections of the party currently blame the other for the loss - which isn't entirely fair truth be told as Lincoln made two unforced errors going into 1864 (namely declaring Kentucky in insurrection and insufficiently catering to the Radicals for fear of alienating moderate Republicans after the Treaty of Rotterdam) but McClellan's path to power was very much a marriage of convenience between two factions in the Democratic Party which are very incompatible. As I hope I've been showing, McClellan really isn't fit for the position of welding those two extremes together (as he was not OTL) and being in the highest office of the land - alongside having a not insubstantial ego - isn't helping create unity.
Let's just say that "interesting times" lay ahead for the US political system.