U 29 Possibly Uncle John Brother Of Alice Noonan

He is a Royal Marine mate or as they were known then (Royal MArine Light Infantry)and you are right he is a marksman.difficult to say which commando unit he was with as they do not have any insignia that denotes the particulr commando regiment
Excellent photo

During World War I, in addition to their usual stations aboard ship, Royal Marines were part of the Royal Naval Division that landed in Belgium in 1914 to help defend Antwerp and later took part in the amphibious landing at Gallipoli in 1915. It also served on the Western Front in the trenches.

The Division's first two commanders were Royal Marine Artillery Generals. Other Royal Marines acted as landing parties in the Naval campaign against the Turkish fortifications in the Dardanelles before the Gallipoli landings. They were sent ashore to assess damage to Turkish fortifications after bombardment by British and French ships and, if necessary, to complete their destruction. The Royal Marines were the last troops to leave Gallipoli, replacing both British and French troops in a neatly planned and executed withdrawal from the beaches. It even required some Marines to wear French uniforms as part of the deception.

In 1918, Royal Marines led the Zeebrugge Raid. Five Royal Marines earned the Victoria Cross in the First World War, two at Zeebrugge, one at Gallipoli, one at the Battle of Jutland and one on the Western Front. After the war Royal Marines took part in the allied intervention in Russia. In 1919, the 6th Battalion RMLI rose in mutiny and was disbanded at Murmansk.

Text via Wikipedia