Mr Kelly, of Summerhill, Co Meath, became an officer of the Irish army at a commissioning ceremony in Defence Forces HQ.
He was joined at the ceremony by his father, Liam, who servesas a detective inspector in An Garda Síochána.
And, four months ago, William’s younger brother, Robert, 19, entered the cadetship for the navy.
“Yeah, it’s a tradition now,” said William. “My younger brother is a cadet in the navy and my dad is a detective inspector in Crumlin and Sundrive [Dublin South].”
William was one of 17 graduating cadets who became officers of the army, along with one equitation officer, yesterday.
The ceremony — held at the training centre in the Curragh Camp, Co Kildare — was overseen by Minister for State at the Department of An Taoiseach and the Department of Defence, Paul Kehoe, accompanied by the Chief of Staff, Lieutenant General Conor O’Boyle.
William said the cadetship was 15 months “very hard training”, adding that it was “tough physically and mentally”, but that he grew in confidence.
He said he spent four years in college studying quantity surveying, followed by a year-and-a-half of work, before getting the cadetship.
He will join his unit in Cathal Brugha Barracks in Rathmines, Dublin, the week after next.
Robert said he spent his first three months in the Curragh and the last six weeks in Haulbowline, the navy’s Cork headquarters, which was “brilliant”.
He will spend the rest of his cadetship — which runs for 24 months — there and hopes to captain a ship one day.