[5] Prior to the founding of the United Irishmen, McCabe was heavily involved in Belfast's liberal and radical community, being a leading figure in the city's anti-slavery circle.
He clashed routinely with the plans of Waddell Cunningham and others to form a Belfast-based slave trading company of which he wrote, ‘May God eternally damn the soul of the man who subscribes the first guinea’.
In April 1791, McCabe resolved with Samuel Neilson, John Robb, Alexander Lowry and Henry Joy McCracken:[1] to form ourselves into an association to unite all Irishmen to pledge ourselves to our country, and by that cordial union maintain the balance of patriotism so essential for the restoration and preservation of our liberty, and the revival of our trade.
In October, the group invited Theobald Wolfe Tone, author of the tract Argument on Behalf of the Catholics of Ireland and his friend Thomas Russell to address a broader meeting.
Fully persuaded of Tone's case that London-appointed Irish executive exploited sectarian division to balance “the one party by the other, plunder and laugh at the defeat of both,” McCabe and his friends formed themselves as the Society of United Irishmen.
[8] Anticipating government repression, two days later McCabe joined a secret directory including Neilson, William Tenant, Robert Simms and Henry Haslett.
[1] As the society replicated among working men and women who there had maintained their own democratic ("Jacobin") clubs, and among tenant farmers long organised in secret fraternities, McCabe and others determined upon a republican insurrection for which they hoped to obtain French assistance.
[9] A Blue Plaque to Thomas and his son, William, was erected by the Ulster History Circle on the wall of St. Malachy's College, Antrim Road, Belfast, which was built on the site of the McCabe home.