In New York, from 1806 he won renown as a trial lawyer representing the abolitionist Manumission Society and disputing race as a legal disability; challenging the conspiracy charges against organised labor; and, in the name of religious liberty, establishing Catholic auricular confession as privileged.

[2] In 1791, when rallying in celebration of the French revolution (Bastille Day), Volunteers in Belfast proposed a political “union” between Protestants of all persuasions and the kingdom's dispossessed and disenfranchised Catholic majority.

[5] As a preface to demands for still greater change, with Thomas Russell he dispensed with the reformer's usual praise for the celebrated charters of English liberty: Magna Carta, the Bill of Rights and Habeas Corpus.

In their Review of the Lion of Old England; or Democracy Confounded (1794) (dedicated to the reputedly enlightened "Empress of all the Russias", Catherine II),[6] they suggested that, honoured in the breach in Ireland, the invocation of these charters did little more than "amuse the masses".

[5]: 647 In The Trial of Hurdy Gurdy (1794), serialised in the Northern Star, Sampson and Russell pilloried the Crown’s suppression of dissent: a barrel organ is charged with playing a seditious tune, Ça Ira.

[1] In Advice to the Rich (1796), he urged the Ascendancy to embrace reform, and prove "by fact and experiment" rather than "idle rhapsodies" that the constitution tended to promote public happiness".

[4]: 48 When on New Years Day 1797, news reached Belfast of a French fleet appearing off Bantry Bay, Sampson addressed an open-air town meeting.

Drennan expressed his astonishment—that Sampson should "leap upon a joint-stool and harangue the populace, at such a time and on such a topic, with such temper, and near such a body of military"—and proposed that he was the "most active" man in Ireland.

[16] But, critically, a key informant was now placing Sampson, alongside O'Connor, at meetings of the Leinster (Dublin) Directory of the movement, and this at a time when the discussion was clearly of armed insurrection.

[1] In April 1798, a month in advance of the United Irish uprisings, Sampson escaped to England, but was returned to Dublin where, under the Banishment Act, he was permitted exile.

There, in March 1799, he was arrested by order of the English minister on the misapprehension that he was the author of a pamphlet circulating in Ireland protesting the country's incorporation in a united kingdom with Great Britain—following the failed rebellion, the government's settled policy.

[1] In New York City, Sampson set up a business publishing accounts of the court proceedings in cases with popular appeal and which advanced arguments for reform.

neither philosophy nor religion have forbade such mixtures.’’[23]: 110 In 1810, Sampson published Trial of the Journeymen Cordwainers of the City of New-York for a Conspiracy to Raise Their Wages,[24] presenting his (unsuccessful) argument in The People v Melvin (1806) for quashing an indictment of unionising workers.

Insisting on the supremacy of the elected legislature, Sampson's objected that the prosecution was reasoning "abstractedly" from principles of English common law without any reference to statute.

[4]: 67–68 in the view that, until acting “counter to the fundamental principles of morality”, men are to be “protected in the free exercise of their religion",[28] the court ruled for the defence, recognising priest-penitent privilege.

[4]: 43 In 1824, he and Thomas Addis Emmet, again supported by Riker,[4]: 67–68  defended Irish weavers in Greenwich Village, charged with riot in a 12th of July confrontation with local Orangemen.

[29]: 1053–1054 In anticipation of Daniel O'Connell's victory at Westminster (the King signed the Catholic Relief Act ending the Protestant monopoly on parliament a month later), on St. Patrick’s Day, 1829, Sampson and MacNeven convened the “Friends of Religious and Civil Liberty” in Tammany Hall.

[38][39] The call for legal reform had not swayed the leaders of Jefferson's Democratic-Republican Party, in whose east-coast city electoral successes he, Duane and other Irish émigrés had played a significant role.

[33]: 153–156 Despite the lack of support for legal reform, Sampson did not follow Duane and other radical elements within the dissolving Jeffersonian coalition into the Democratic Party of Andrew Jackson.

[4]: 75  He was overwhelmed by the ability of the Tammany Hall Democratic Party machine to deliver the greater part of the growing Irish, and broader immigrant, vote.

They found themselves denounced by the community's principal paper, the Irish Shield, as presumptuous and oligarchic and for failing in their gratitude toward Daniel O'Connell, the "Emancipator", to whom a monument was truly due.

[47] The white marble tomb, erected by his wife, Grace, and daughter, Catherine, bore an inscription describing him as "An United Irishman [who] defended the cause of civil and religious liberty".