The 21-year-old from Moyross, County Limerick, has made serious progress in the senior ranks since winning a Grand Slam with the Ireland Under-20s in 2007.
2008 saw him break into the Munster first team - showing his versatility across the province's back-line - he continued his development with the Ireland 'A' side and scored a try with his first touch to crown his Ireland senior debut against Canada.
In 2009, he picked up a Six Nations winner's medal (although he failed to get any game-time during the Championship), and he finished as the joint-second top try scorer in the Magners League as Munster secured the title.
Ronan O'Gara says he has never seen a player 'improve so much and so quickly' as Earls has, and the Lions tour is sure to bring on the centre-cum-full-back even more.
Speaking about his first Lions selection, he said: "I didn't even think that I would have an Irish cap at the end of this season. It's my first professional season and I was just hoping to get a few starts with Munster.
"A couple of games went well in the Magners League and I got a few starts in the Heineken Cup. But not in a million years did I think I would be on a Lions tour."
His father Ger Earls, the highly-regarded Young Munster flanker of the 1980s and 1990s, left a lasting impression on Lions captain Paul O'Connell when the pair played together for Munsters - O'Connell as a gangly teenager.
O'Connell's admiration for Ger clearly remains. In the 2009 Lions media guide, O'Connell answered 'Ger Earls' when asked who his first rugby hero was.
Unlucky not to be capped by Ireland, Ger has watched his son's career flourish in such a short space of time.
Sometimes he might admit to being fearful, as any father would be, watching their son play against huge men, who are almost double Keith's size and vastly more experienced.
Ger said recently: "I'd say to Keith, 'do you ever get anxious before these matches?' and he'd say 'nah'." Even if he is nervous, outwardly the Limerick youngster portrays a confidence and rugby ability that has made the northern Hemisphere sit up and take notice.
Now the southern Hemisphere will get its first up-close glimpse of the shaven-headed 'wunderkid' and with O'Connell and O'Gara, two of his Munster 'minders' in Saturday's side, Earls is looking forward to donning a red jersey of another kind.
"I'm delighted to be making my debut for the Lions, and even more so because Paul will be leading the side out.
"I've known Paul since I was eight-years-old and he was playing in the same Young Munster side as my father.
"I have the greatest respect for him and he is a guy you would do anything for on and off the pitch.
"There was no better man to look to for an example as a young lad growing up. He is the captain of the Lions and one of the best rugby players in the world but nothing goes to his head.
"I'm probably the wild card in the party. My aim is to train hard and give myself the chance to play in a Test match.
"There are good players around, like Brian O'Driscoll and Paul, and I know it's going to be tough. But I'm here to give it my best shot.
"My dad has been on the phone and his advice was to go out to play my own game and relax. Just treat it like a week-in- week-out game and enjoy it."
Earls' direct opponent this weekend, Deon van Rensburg, is one of a small number of Royal players to have played Super 14 rugby. Both players will be hoping to impress and win their individual match-up.
"I've seen a few clips of Deon and he looks explosive and a strong looking guy. We're both in a similar position - it's his chance to prove himself and it's the same for me on the first match on tour," he explained.
"We don't know much of the Royals as a whole, but there are a couple of guys with Super 14 experience for the Cheetahs and Lions. They will be up for this, it's their only chance in 12 years to play us."
Earls will be partnered by Wales' Jamie Roberts in one of the Lions' youngest ever midfield combinations. Roberts is just 11 months older than Earls.
If things go right, the tour could start with a bang for both players.
"This is a good opportunity for me. I don't want to get the tag of a utility player," Earls said.
"I want to nail down a position and I would like it to be at 13.
"Of course you would play anywhere for the Lions or your country, but I'm not keen to be known for my versatility.
"I have a chance on Saturday to play in my favoured position and show what I can do. Jamie is different from my inside centre at Munster, Lifeimi Mafi. He is big and powerful and good to play off. I think we will hit it off."
Cardiff Blues powerhouse Roberts, one of seven Welsh players in this weekend's Lions team, is similarly determined to make a good first impression.
He said: "I'm excited about the partnership because we are different sorts of players. We have gelled well in training and the first thing that struck me about Keith was just how quick he was.
"He's such a talented player, he's absolutely lightning. I didn't realise how quick he was until training this week.
"It's a chance to stake a Test claim and we're all chomping at the bit."