Recalled as adjutant of the 4th King's Regiment, Harington served with the battalion in Ireland for about a year under the command of Lieutenant Colonel Grattan (his future father-in-law). He was reassigned, with Colonel Grattan, to the 13th Provisional Battalion after the disbandment of the 4th and later rejoined the 2nd King's at the Curragh as adjutant from 11 September 1902. Harington returned to Sandhurst as commanding officer of Gentleman Cadets in 1903. The appointment lasted almost four-years and entailed him being responsible for the instruction of officer cadets. He was selected for employment at Staff College, Camberley in 1906 – a decision that had neither been anticipated or sought – and later moved to the War Office and Army Headquarters. Harington transferred to Aldershot in 1911 to assume the position of brigade-major to the 6th Brigade, which consisted of the 1st King's and five other battalions.