"Nevertheless, he refused to sleep as usual near the golf course to be ready the following day. Instead, he drove back to New York to see Meghan. His performance suffered," Bower writes.
Bower's book claims Markle then raved about McIlroy on her blog, The Tig, prompting her then-boyfriend, chef Cory Vitiello, to ask if she was cheating on him. Markle said her time with McIlroy was innocent, and "he believed her".
A couple of months later, Markle and McIlroy were seen enjoying dinner together in Dublin, with a newspaper gossip columnist saying she looked "smitten".
According to Bower, Vitiello again asked Markle about the nature of her relationship with McIlroy and she again insisted they were just friends.
The couple remained together until 2016, with Bower's book claiming Markle was still living with Vitiello when she went on her now famous blind date with Prince Harry in London that year.