After attending school at Waterford, in 1857 Ardagh entered Trinity College, Dublin, intending to follow his father into the church.

[2] After training at the Royal School of Military Engineering at Chatham, Ardagh supervised the construction of Fort Popton, one of the new defensive works at Milford Haven, built under the terms of the Defence Act of 1860.

[1] Early in the American Civil War, when tensions between Britain and the United States where raised as a result of the Trent Affair of November 1861, Ardagh was sent to the colony of New Brunswick to construct a military telegraph line to the St. Lawrence river.

[1] In February 1871, Ardagh witnessed German troops entering Paris following France's defeat in the Franco-Prussian War.

After recovering from an attack of fever, he resumed his duties at the War Office in April 1877, when he completed a report and survey begun in the previous year on the sea defences of the Lewes and Laughton Levels.

[1] From December 1877 to March 1878 Ardagh was in Italy on special service for the Foreign Office, and then attended the Congress of Berlin as a technical military advisor to General Sir Lintorn Simmons.

He was mentioned in Lord Wolseley's despatch at the end of the campaign, and also received the British war medal with clasp for Tel-el-Kebir, the Khedive's bronze star, and the fourth class of the order of the Osmanieh.

[1] In February 1884 Ardagh, as Commanding Royal Engineer and chief of the intelligence department, accompanied the British force under Sir Gerald Graham, which was sent from Cairo to the Eastern Sudan.

[10] In October 1884, when an expedition to relieve Khartoum was organised, Ardagh favoured an approach overland via Suakin and Berber, but the commander Lord Wolseley resolved to travel by boat up the Nile.

His energy, devotion, and quiet cheerfulness helped to expedite the fatal enterprise, and at the end of the disastrous campaign[1] he was promoted to brevet colonel (25 August 1885),[11] receiving the Order of the Medjidieh (3rd class).

Meanwhile a copy of Military Notes on the Dutch Republic, a study prepared in secret by Ardagh's intelligence branch, fell into the hands of the Boers after the Battle of Talana Hill on 20 October 1899, and was later published.

He returned to South Africa later in the year with the temporary rank of lieutenant-general as member of the royal commission for the revision of martial law sentences.

[1] On 9 August 1902, at the age of 62, Ardagh was placed on half-pay and retired from active military service,[18] but was still employed by the Foreign Office.

He succeeded Lord Pauncefote on the Permanent Court of Arbitration at the Hague, and was appointed by the British government as a director of the Suez Canal Company.

[1] Ardagh died on 30 September 1907 at Glynllivon Park, Carnarvon, and is buried at the Church of St Mary & All Saints, Broomfield, near Taunton, Somerset.

[1] On 18 February 1896, Ardagh married Susan, the widow of the 3rd Earl of Malmesbury and the daughter of John Hamilton of Fyne Court, Somerset.