The brave 16-year-old won the hearts of the nation when he made a heartfelt plea to young people to treasure life.
He died at his home in Co Kerry surrounded by his loving family.
His heartbroken father, Fionnbar, said his son was now at peace.
“His pain is over now,” Mr Walsh said.
Donal came to national prominence after he spoke publicly about his own illness and how angry it made him feel when he heard of young people taking their own lives when he had no choice about when his ended.
In October, doctors advised him to celebrate an early Christmas but Donal continued to defy them to the end.
Dad Fionnbar said Donal's life-affirming message has already saved lives, and Donal had been inundated with messages from people thanking him for saving their lives or the life of a loved one.
"We've received letters and messages from people all over the world who have been inspired by what Donal had to say and have been able to resolve their problems," he said.
The courageous 16-year-old received nationwide admiration after he appeared on the Saturday Night Show in March.
Donal has been fighting cancer over the past four years but went on TV because he wanted to speak out about young people taking their own lives.
The rugby fanatic, from Blennerville, Co Kerry, appealed to youths to ask for help.
"I feel angry these people choose to take their own lives, to ruin their families and to leave behind a mess that no one can clean up," Donal said.
"Yet I am here with no choice, trying as best I can to prepare my family and friends for what's about to come and leave as little a mess as possible" he said.
Just weeks before his death, he made a recording of his anti-suicide message, which is going to be circulated to schools.
Tributes have been pouring in following Donal's passing. Chairman of Tralee Rugby Club, Adrian Gray, said Donal was central to the club, which was devastated by his death.
"We're in shock. No matter how much you expect it, when it arrives, it's just devastating," Mr Gray said.
Kerry footballers Paul Galvin and Killian Young joined in the tributes. Mr Young described Donal as a "pure inspiration for life", while Mr Galvin said he was "a true inspiration for everyone in Tralee and beyond".
Arts Minister Jimmy Deenihan said Donal had exceptional strength and courage that would continue to inspire other young people, adding: "He especially showed tremendous bravery in his final weeks when he gave that strong message to young people that they should value life."
Former Cork goalkeeper Donal Og Cusack wrote: "Sad to hear about this passing... inspirational outlook... lesson to all of us... live while we can."
Dublin football ace Dean Rock said that Donal "set an example to everyone in this world to live life to the full and enjoy what you have".