In 1792, following Bastille Day celebrations, his attempt to water down a town resolution in favour of political coalition with Ireland's Catholic majority was defeated by fellow Volunteers styling themselves the United Irishmen.

His merchant ships carried rough linen clothing and salted provisions from Belfast to the British West Indies, where they were sold to slave plantations; sugar and rum from the West Indies to Baltimore and New York City; and flaxseed from the Thirteen Colonies (where, in contrast to Belfast and its hinterlands, the relative scarcity of labour made it unprofitable to process flax into linen fibre) back to Ireland.

Benefitting from the rise in the prices of provisions during the Seven Years’ War (1756–1763) and, although themselves trading illicitly with the French and Spanish, from licence to attack and plunder enemy vessels, Greg & Cunningham became one of New York's largest shipping companies.

Cunningham was a delegate to Volunteer conventions in Dungannon and Dublin which echoed American discontents in calling for legislative independence and freedom from the restrictions of Britain's Navigation Acts.

[4] The prosperity of Belfast was heavily invested in trade with the West-Indian plantation economy, but for public opinion in the town the actual carriage of human chattel proved a step too far.

[1] A rare victory for a Presbyterian, the result was overturned by a committee of the House of Commons on the grounds that Cunninghan's Belfast supporters had exercised undue influence on his behalf.

[13] On Bastille Day 1792, celebrating what he and his fellow Whig reformers still regarded as a French reprise of England's Glorious Revolution, Cunningham led his Volunteers in a muster and parade in Belfast.

Samuel Neilson, publisher of the Painite paper, the Northern Star, found Cunningham the previous evening in an inn haranguing Volunteers up from the country against Catholics "and talking of some sedition to be broached next day".

[14] The Address's offending passage proved to be the declaration that "no reform, were even such attainable, would answer our ideas of utility or justice, which should not equally include all sects and denominations of Irishmen".

Cunningham, together with his church minister, William Bruce, and the publisher of the Star's rival title, The News Letter, Henry Joy, proposed, rather, "the gradual emancipation of our Roman Catholic brethren".

"[15] As the United Irishmen prepared, under growing martial-law repression, for a republican insurrection, Cunningham declared his loyalty to the Crown and to the government in Dublin, now entrusted to the least compromising representatives of the Anglican Ascendancy.