In June 2014, Ronan fell 20ft from the top of a telephone box in Brighton during a night out with friends.
At the time, he was taken to a nearby hospital in a critical condition and was later transferred to a specialist neurological centre.
Ronan was the youngest of Liam’s sister Bernadette Sexton’s six children.
Parish priest Father Luke McWilliams told the Belfast Telegraph, who first reported the news, the family had asked for privacy as they mourn their loss.
Ronan’s death comes nearly 10 years after Liam’s wife, Natasha Richardson, tragically died after she hit her head during a skiing trip in Quebec.
The 45-year-old ‘Parent Trap’ actor was not wearing a helmet at the time of her accident in March 2009.