[2] A moderate constitutional nationalist and supporter of the Irish Parliamentary Party, Sullivan was a prominent campaigner for the recruitment of Irishmen into the British army during the First World War.
His opposition to Sinn Féin republicanism and his prominent role in conducting prosecutions on behalf of the Crown during the Irish War of Independence led to an attempt on his life in 1920.
In April 1921 Derry House, Rosscarbery in County Cork, which Sullivan had purchased two years previously was burned by the IRA.
As the facts relied on by the prosecution were largely undisputed, Sullivan limited himself to arguing a technical defence that the Treason Act 1351 only applied to acts committed "within the realm" and not outside it; this, in spite of Casement's exhortations to consider "the wider issues" in his arguments, and the opportunity of citing (chief prosecutor's) F. E. Smith's own declaration of treason in the preceding years alongside fellow Ulster Protestant leader Edward Carson.
[7][page needed] The Act's terms had however been expanded by case law over the previous 560 years, and the defence was rejected by the trial judges and by the Court of Criminal Appeal.