Explanation: After the US Civil War, there were a few instances wherein Irish-American combat veterans on the Canadian border started a little bit of trouble, including an attempt to invade Canada to pressure Britain to withdraw from Ireland
Before the US Civil War, the Mexican-American War broke out. This is widely considered, including by contemporaries, to be one of the least justified major wars the USA ever engaged in. For this reason, many Irish-American troops deserted to serve the Mexican cause in Saint Patrick's Battalion.
Funny enough, after the US Civil War, US troops on the southern border would be 'unofficially' involved in gunrunning to Mexican Republican movements against the European-imposed monarchy (which had invaded Mexico during the US Civil War to overthrow the Mexican government).