Hooker Best, who started all five of Ireland’s Six Nations fixtures, was added to the squad on Sunday just 24 hours after Ulster lost the PRO12 final to Leinster.
“It was hard when I didn’t make the initial squad because it is everyone’s ambition to play for the Lions, but to be selected now is a massive honour,” said Best.
“To be finally selected makes playing professional rugby worthwhile and makes up for the feeling I had a few weeks ago when I thought the chance had gone.”
Meanwhile, Munster back row Peter O’Mahony has been made captain for Ireland’s summer tour of North America in Best’s absence.
The 23-year-old O’Mahony has impressed for Rob Penney’s side this season and during the autumn internationals and the Six Nations, he made himself a mainstay of the Ireland back row.
He will be Ireland’s youngest captain since Brian O’Driscoll led the men in green on their tour of Australia in 2002.