The 21-year-old was a replacement as Scotland defeated England 25-13, but will now start against the Irish after winger Tommy Seymour was ruled out through injury.
"We were just at the team meeting and [head coach] Gregor [Townsend] pulled me aside and told me," Kinghorn said.
"It was a big surprise but a chance I am really excited for. I quickly let my parents know, but I've not looked at my phone just yet."
Kinghorn will have to switch to the wing from his more familiar role of full-back, but is well up for the challenge of coping with the kicking game of Conor Murray and Johnny Sexton.
"That's just expected with their gameplan and the role of the winger is to diffuse that," he added.
"We've been working hard on all our skills throughout the week, so come game time, we'll be ready.
"I feel quite comfortable on the wing. I've run in there a couple of times, been training there most of the time I've been with the squad.
"I'm looking forward to it; hopefully it will be a good game."