Unfortunately for the Fenians, the British government in Ireland (usually referred to simply as "Dublin Castle") had a superlative G-Division (domestic intelligence). They had agents everywhere, with plenty of money to spread around, and they knew just how to take advantage of the Irish penchant for drink and loquaciousness. Most of the Irish veterans returning from America were expected, and quietly picked up soon after they arrived. To ease already strained relations with Washington, the Brits simply deported most of them back to the States -- where many of them joined in the Fenian incursions into Canada..
The Fenians had an ambitious but not very well thought-out plan, including subverting some of the British Army's Irish regiments stationed in Ireland. But, between squabbling factions that couldn't agree on timing or leadership, and British foreknowledge (most of the leaders were arrested shortly before the rebellion was to start), the Fenian Rising of Feb./March 1867 fizzled quickly.
They left a lot of fine songs about "Bould Fenian Men," and a hard lesson that was well-learned by the leaders of the 1920-21 War of Independence.