It expressed their joy that the Americans had succeeded in throwing off “the yoke of slavery” and suggested that their exertions had "shed a benign light on the distressed kingdom of Ireland".

While the new MP reacted to revolutionary events in France, and to the prospect of war with the new republic, by rallying to the government, Birch was persuaded that reform would have to be sought in an extra-parliamentary union with the Catholic majority.

In 1792 Birch joined the United Irishmen, intervening with them in a crucial Bastille Day Volunteer debate in Belfast to defend a resolution in favour of immediate, unqualified, Catholic emancipation.

He decried the "withholding of rights from our Catholic brethren" as "criminally unjust and impolitic", and declared he would rather transport himself to Botany Bay, "than live in a country which continued to keep itself in abject slavery, by its internal divisions".

Birch convened the Saintfield Society of United Irishmen and on Christmas Eve 1792 moved their first resolutions.Resolved, that we will steadily pursue every reasonable, legal and constitutional means in our power, to obtain a more equal representation of the people in Parliament and a shorter period of parliamentary delegationResolved that a radical reform can never be affected, but by extending the right of suffrage to all sects and denominations of Irishmen.Resolved, that we look upon our brethren Roman Catholics as men deprived of their just rights--that we highly approve of their present mode of proceeding and sincerely and heartily wish them success.

The Belfast News Letter (4 January 1793) reported that the congregation unanimously applauded a proposal that "for the defence of their families and properties" a further 500 of their number "be added to the National Guards [the Volunteers] of Ireland".

At the same time Birch asked his congregants to consider that "we live in a very advanced and enlightened period of the world, when ignorance and superstition are falling like lightening from heaven" and that, as a minister, he had a duty to bear witness against the corruptions of government.

[9] In 1797, eleven of Birch's congregation were charged with attacking the house of the McKee family, local loyalists who supplied the authorities with such information as they could gather on the activities of United Irishmen.

According to witnesses at his subsequent court-martial, the day after the Battle of Saintfield, "Pike Sunday", Birch appeared among the rebel army assembled at Creevy Rocks, a hill outside the town.

None testified to his preaching a sermon, but there is at least one record (possibly spurious) of his offering the following:[14]Men of Down, we are gathered here today ... to pray and fight for the liberty of this Kingdom of Ireland.

The Letter was addressed to an American audience and sought to counter Federalist propaganda in which the rebellion in Ireland was discredited as part of a larger effort to generate and sustain alarm over revolutionary developments in France.

Birch did not restrain himself from proposing that in this resolve, the people of Ireland "are inspired (as they think) with a well-grounded belief, and hope that the time is arrived, when the [Biblical] Prophecies concerning the Universal Dominion of Christ's Kingdom, and the peaceful happy state upon earth ... are to be fulfilled".

[23] In the United States, Birch returned to the ministry, first in Philadelphia and then, unhappily due to various disputes, political and religious, with the Ohio Presbytery in Allegheny County, western Pennsylvania.

Despite his own millenarianism (based on his reading of the Books of Daniel and Revelation he concluded that the Second Coming of Christ would occur in Washington in 1848), as in Ireland Birch robustly defended the Presbyterian orthodoxy.

In a broadside published in 1796 he had denounced the excesses of their outdoor communion observations, suggesting that their primary aim was "large collections", and had rebuked their reactionary politics.