After a promising Army career in the American Civil War he joined forces with the Fenians and later established an Irish colony in Nebraska where today the city of O'Neill, Nebraska stands as a memorial to his endeavours on behalf of the Irish communities in the United States of America.
Michael Atkinson, Attorney-General of South Australia, spoke of those members of the ALP who wished to remove the title Queen's Counsel and other references to the crown as "Fenians and Bolsheviks" in a speech given at the ALP Convention in Adelaide on 15 October 2006.
In 1866, Dennis led an ill-fated militia attack against the Fenians at Fort Erie.
In 1863, Fennell became one of the first Fenians recruited from County Clare; he then helped recruit and organize a group of men in preparation for the Fenian Rising.