She goes on: "I hope you had an amazing life I absolutely love you and my life could not of been this amazing without you!"
O'Brien took to Twitter to share the adorable messages with the world.
He wrote: "My youngest has had my old phone for a couple of years. Just for games, which I download for her before disconnecting the internet.
"Still has my old contacts though & it turns out she's been messaging my dad, who died 5 years ago.
"I may have something in my eye."
Months later, his daughter sent another message to her granddad saying that she is nearly 10 and that her sister Elizabeth is 12.
His photo has been liked more than 124,000 times in less than 24 hours.