Also, the Fenian Brotherhood, an Irish-American revolutionary (terrorist) group in the late 1800s. It was formed predominantly from former Union soldiers of Irish-American descent. They hoped to tie up British forces in North America whist their Irish comrades fought to liberate Ireland.
On June 1, 1866 a Fenian force of some one thousand men with an experienced commander named John O’Neill crossed the Niagara river, spoiling a picnic. Things got more serious a day later when a force of Upper Canada volunteers engaged the Fenian force in battle at Ridgeway, on the north shore of Lake Erie. The Fenians may have been advancing towards the Welland Canal, hoping to disrupt shipping, but they got word of the volunteer response and lay in wait for it at Ridgeway. A lack of experience and some poor command decisions worked against the defenders. Mistaken identity, general confusion, and a false report of cavalry caused both the volunteers and a British army unit to panic and fall into a general retreat west to Port Colborne. The Fenians, cut off from resupply, retired to Fort Erie, from where they subsequently returned to the US. This was the only major Fenian incident.
Ultimately, Fenian attacks in Canada had little or no effect, but the threat of further action may have played a part in Canadian confederation some years later.
Nonetheless, my Mom called me a "little Fenian" when I was naughty, so at least the usage survived long after the actual threat collapsed.