The 1918 South Armagh by-election was held on 2 February 1918. The by-election was held due to the death of the incumbent Irish Parliamentary Party MP, Charles O'Neill. It was won by the Irish Parliamentary candidate Patrick Donnelly.[1]
| Party | Candidate | Votes | % | ±% | |
|---|---|---|---|---|---|
| Irish Nationalist | Patrick Donnelly | 2,324 | 63.34 | −10.90 | |
| Sinn Féin | Patrick McCartan | 1,305 | 35.57 | New | |
| Ind. Unionist | Thomas Wakefield Richardson | 40 | 1.09 | New | |
| Majority | 1,019 | 27.77 | −20.71 | ||
| Turnout | 6,345 | 57.83 | +0.66 | ||
| Irish Nationalist hold | Swing | ||||