The Fenians were members of a branch of the Irish Republican Brotherhood called the Fenian Brotherhood in Ireland, who were an Irish Republican organization founded by John O’Mahony and Michael Doheny. The organization was dedicated to free Ireland from the hands of Britain and become independent. Their job was to attack British colonies in North America to help convince Britain that Ireland needed to be on its own, while their Irish counterparts rioted in Europe.
The Fenians believed that that Ireland has aright to independence, and that this right could only be won by an armed revolution. They dispersed themselves across British North America, and rioted and raided to hopefully make Britain realize that they would have nothing short of independence. A famous group of raids, known as the Fenian Raids in Canada occurred 1866-1877.
The Fenians were not totally focused on stopping Confederation. They were more focused on causing trouble during a tricky time for Canada, and drawing more attention to themselves. In the end, they actually encouraged early Canada to confederate by posing a threat to the colonies. We could almost thank them for helping early Canadians to agree with each other!