The Mid Antrim seat existed from 1885 to 1922. It had three MPs, all called O’Neill. The first, Robert Torrens O’Neill, was defeated by William Pirrie Sinclair in a by-election for the previous seat, Antrim, in May 1885. O’Neill gained the new seat in the general election in November that year.
When Robert died in 1910, his nephew Arthur O’Neill took over. Arthur had fought in the second Boer war and had been involved in the relief of Kimberley. He served with the British army’s oldest regiment, the Life Guards, and was killed near Ypres in November 1914. He was the first MP to die in the First World War. His brother, Hugh, was unopposed at the subsequent by-election.