The death has been announced of five-year-old Kerry boy Liam Heffernan, whose family set up the Bee for Battens charity.
Liam had the rare neuro-degenerative condition Battens Disease.
His father Tony Heffernan tweeted this morning: "LittleLiamo has grown angel wings & joined sister Saoirse in heaven."
Mr Heffernan and his wife Mary lost their first child Saoirse to the same disease in 2011. She was also five.
Just weeks after Saoirse's death, Liam travelled to the United States to undergo pioneering brain surgery, in a bid to prolong his life.
Mr and Mrs Heffernan set up the Saoirse Foundation and their Bee for Battens charity has helped raise awareness of the rare disease, and raise funds for research and support for other families.
The charity plans to develop Ireland's first respite centre for families of children with rare diseases.