O'Neil was brought to trial some time after by the United States Government, on a charge of violation of the Neutrality Laws, and was sentenced to six months' imprisonment.

This was a hard blow to the Fenian organization, and it gradually went to pieces.
But the warlike spirit had not died out in O'Neil, and he began to plan new ideas.

His hatred of British institutions appears to have been so deep-seated that he was willing to sacrifice not only his liberty, but life itself, to undertake any scheme that had for its object their overthrow, and it was not long before he was again implicated in a plot against the Dominion of Canada.
Shortly after his release from prison in 1870, he entered into a conspiracy with emissaries of the rebel Louis Riel to assist in a great uprising in the Canadian Northwest, in which the Indians and half-breeds were to be utilized.

O'Neil was ready for anything, and consented to invoke Fenian aid in conjunction with Riel's rebellious plans, by participating in an invasion of Manitoba.

He managed to obtain a few hundred stands of breech-loading rifles and a quantity of ammunition that had escaped seizure by the United States authorities at the time of the Fenian Raid of 1870, and with the assistance of Gen.