This article seeks to contextualize a rare piece of evidence of the Catholic Church's attempts to control nationalist political expression among Irish migrants. The evidence, a letter from a priest to his bishop in Darlington, was generated by an investigation of a street riot in Sunderland in 1858. A detailed statement of such controlling influences is uncommon, even though historians have occasionally uncovered fleeting examples that are similar in nature. The discussion which follows seeks to fit this evidence, and its immediate context, into a wider historiography concerning the interplay of social Catholicism and the political involvement of Irish migrants. This document portrays the English priest as a kind of politico-religious policeman, and explains the lengths to which the Church was willing to go in ensuring that strict adherence to Catholic practice was not affected by the demands of clandestine political organizations. Although the events discussed here are very specific, in both period and place, the article seeks to contribute to an understanding of parish life where politics and faith became entwined.