The SDLP and Sinn Féin regarded the resignations as a publicity stunt, and were reluctant to take part in the resulting by-elections. In the event, they contested only the seats which they believed to have an anti-unionist majority.
In four constituencies, no political party was willing to contest the by-election. This effectively made these the last uncontested by-elections in British history. However, to ensure that there was a contest and the Unionists would be able to demonstrate their point, Wesley Robert Williamson changed his name by deed poll to "Peter Barry", Peter Barry being Minister for Foreign Affairs of the Republic of Ireland. "Peter Barry" stood in these four constituencies under the label "For the Anglo-Irish Agreement", allowing a contest, but did not campaign.
The unusual circumstances led this to be the greatest number of UK Parliamentary by-elections ever held on a single day.
The sole exception to this pattern was the Newry and Armagh by-election, where Seamus Mallon of the SDLP was able to take the seat. Former Cabinet Minister Enoch Powell was able to narrowly survive a strong challenge from the SDLP in South Down and was subsequently defeated at the following year's general election. In the western constituencies of Mid Ulster and Fermanagh and South Tyrone the Unionist candidates were able to survive with less than 50% of the vote due to a split Nationalist vote and both seats were gained by Sinn Féin in later elections.
The results of the fifteen by-elections were cited by Unionists as a rejection of the Agreement by the Northern Irish electorate, but did not succeed in repealing it.