The question was sparked when I learned that a group of Irish-American officers did return to Ireland in the 1867 Fenian Rising, and the the Fenians had apparently wanted or intended to gather many of these Irish veterans. While the Irish may have been just a convenient manpower pool to the US government, did the British government see the creation of a large number of experienced soldiers that could form the nucleus of a professional Irish army as a threat?
As far as they were concerned, they were emigrants, and wouldn't be coming back. Britain kept a close eye on the CW, but saw it mainly as a rival damaging itself and let it happen. In 1861-65 the British government had no idea that the Irish would rise up in 1867, nor that the Irish emigrants would return having been trained in soldiering.