The 37-year-old actor and girlfriend Tamsin Egerton welcomed a daughter, the first child for both, last year.
The pair are doing their best to keep details about their baby private, but Hartnett opened up about fatherhood in an interview on television show "Good Morning Britain".
"It changes everything," Hartnett said of becoming a father.
"What can you say, it's where the rubber finally meets the road and everything you've done beforehand is a preamble and suddenly you're actually living your life for the first time.
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