Donal, a big Munster fan who died of cancer two years ago, would often console Paul about matches he had lost.
The 16-year-old from Blennerville, Co Kerry, who wanted to play for Munster, also spent his short life appealing to teenagers not to be tempted to take their lives.
O’Connell is preparing for Ireland’s clash with Scotland in Murrayfield in Edinburgh on Saturday.
Last weekend had been expected to be a milestone for the captain when he got his 100th cap, but an unyielding Welsh line ruined a potential Grand Slam party.
In an interview with Woman’s Way, O’Connell said Donal was the most interesting person he ever met.
“To chat to him was to be in the presence of a special person,” he said.
“He’d console me about matches I had lost and tell me to keep the faith, that it would come right, while at the same time going through his own battle.”
O’Connell said that Donal’s only concern when he was dying was how his passing would affect his family and friends.
“His mental strength and his intelligence blew me away,” said O’Connell.
On a lighter note, O’Connell said his earliest memory was watching Bosco on television in the morning while eating Liga.
The big man also confessed that his greatest vice is a sweet tooth.
“It’s fine if they are not in the house but if they are and I know they are it will eat away at me until I raid the press and demolish them,” he said.
He revealed his guiltiest pleasure is eating a Bounty with peppermint tea while watching Grand Designs, a TV series about people building their dream home.
He also admitted being “addicted” to American political drama The Good Wife.
Before the birth of his children, his happiest memories would have been rugby ones, he told the magazine.
“Winning the Grand Slam after years of failing was pretty special,” he said.
Nowadays, his happiest memories are those that involve his wife, Emily, and children Paddy, 4, and Lola, four months.
He loved those times spent with the family after a match — preferably one that the team had won.
“I think, looking back, they will be, and are, my fondest memories,” he said.
While “there was not a whole lot” that made him angry, he did get annoyed when the team did not play to the best of their potential.
“I’ll go with that: unfulfilled potential,” he said.
Despite his achievements, Paul said he hoped most of all to be remembered as a good father and husband.
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