US lacks a navy in 1812
I beg to differ
not counting the US Navy, a total of 500 letters of marque were issued during the war to US privateers. They took 400 prizes in the last year, about 300 a year on average during the 3 years of the war (destroying about 10%, losing about the same back to the British, but selling the rest)
this quote is interesting
" On Sept. 30, 1814, Lloyd’s of London reported that two American warships and several privateers had captured 108 British prizes that month. Maritime insurance rates stood at triple and quadruple the rates of 1811—if insurance could be found at all. Hard pressed by merchants, ship owners and a people tired of years of war, the British government joined the Americans at the treaty table in Ghent, Belgium, in late 1814, a year in which losses to American privateers neared 400 vessels."
http://www.historynet.com/war-of-1812
that was pretty much a repetition of losses suffered by the British during the Revolution as well
In other threads I have posted how many actual steam powered warships the US Navy had in 1861 compared to the Royal Navy. Basically it was roughly half the size in cruisers and sloops, which are your raiding types, and the US has the ability to break a local blockade with ironclads after 1863 for short periods to allow raiders out.
That counts as a Blue Water Navy
that said, there is almost no political support in the United States without a British intervention in North America to help the Irish under just about any circumstances I can think of
"Irish need not apply" is a myth, but only just.
The US is far more likely to go looking for a foreign war against the Spanish (Cuba etc) or to steal more of Mexico
But a fully mobilized US could be a serious threat to Spain...but doing anything in the British Isles or France is a bit unlikely in terms of capability