He was returned unopposed as one of six Sinn Féin candidates at the 1921 elections to the new House of Commons of Southern Ireland, which had been established under the Government of Ireland Act 1920. In common with the other Sinn Féin members elected, he did not take his seat in the short-lived new Commons, sitting instead in the revolutionary Second Dáil.
Derham was re-elected as a Pro-Treaty Sinn Féin candidate at the 1922 general election, and as a candidate for the new Cumann na nGaedheal party at the 1923 general election. He died suddenly aged about 35, less than three months later, in November, triggering Dáil Éireann's fourth-ever by-election, which was won on 19 March 1924 by the Cumann na nGaedheal candidate, Batt O'Connor.
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