In the early years of the American Civil War, military protocol allowed men back on the field after a battle to retrieve the bodies of officers for proper burial, enlisted men were removed for mass burial. During these times of truce, it wasn't all that unusual for Irishmen of both sides to discuss the battle in terms of what they had learned and how it could be used against England after the war. Together, these Irish born and Irish-American soldiers made up the largest group of forces in the War between the States. More than 100,000 of these men had come to support the Fenian movement which had started a few years earlier in places like Cork and Tipperary in the southwest of Ireland.

While O'Donovan Rossa and James Stephens led the Fenians in Ireland, John O'Mahoney headed up the American contingent, raising funds with the issuance and sale of Fenian bonds which would have been redeemed upon the success of a military rising against occupying English forces in Ireland. Fenian plans eventually led to an invasion of Canada in an effort to weaken the British Empire. In April, 1866, O'Mahoney attempted to capture Campo Bello off the New Brunswick coast; this was followed by the May battle on the Canadian shore when 800 Fenians took Fort Eire. The American military eventually followed the Fenians into Canada, and rather than fight their American army, the Fenians left Canada.

1867, saw former Confederate and Union troops travel to Ireland and to assist in the rising. Yet only 4,000 of the 30,000 Fenians were armed, leading to their defeat at the hands of their English military oppressors. The 1866 $10 Fenian bond measures 10½" across by 6¼" and were signed by John O'Mahoney. They were redeemed almost eighty years later when Eamonn De Valera called the Fenian bonds in. Any outstanding bonds were thereafter valued only as collectibles, which today rarely show up at shows or auctions.