Although not much is known of his schooling and early years, he evidently received a high degree of formal education, as illustrated by his knowledge of history and oratory skills which he displayed at a young age.

In January 1878, Brennan organised a homecoming reception for several recently released IRB members, including Michael Davitt, who had been imprisoned at Dartmoor for seven and a half years.

[1] The three men fervently advocated for widespread agitation in the west of Ireland, where the conditions of tenant farmers were especially poor, igniting a period of civil-unrest and sporadic violence known as the Land Wars.

[3] In May 1882, Parnell agreed to the Kilmainham Treaty, in which he withdrew the manifesto and pledged to bring violence to an end in exchange for government leniency on rent owed by over 100,000 Irish tenant farmers.

In order to prevent further fragmentation amongst Irish nationalists, Parnell invited Brennan and Davitt to draft a programme for a new national political organisation to replace the Land League during the summer and autumn of 1882.

[4] He emigrated to New York in late 1882 to work for Patrick Ford's Irish World newspaper, which had been the largest fund-collector for the Land League and promoted radical views in opposition to Parnell.

[5] Despite his close contacts with Clan na Gael, Brennan opposed the Fenian dynamite campaign of the 1880s on the grounds that it harmed the reputation of the Irish republican movement.

All right-thinking men would deplore the necessity of having recourse in this country to scenes such as have been enacted in other lands, although I for one will not hold up my hands in holy horror at a movement that gave liberty not only to France but to Europe.

It is necessary to base our fight on the true principles, and that is to nationalise the whole soil of Ireland - that of the town as well as that of the country - by taking what is now paid in rents for the common benefit of the people.