Weight: 88 kgs (13st 12lbs)
Leinster appearances: 47
Why he's so important? O'Brien demonstrated his versatility to match ability last season playing fullback, centre and wing with aplomb and during the present campaign has played in the 11, 14 and 15 jerseys. He became the first player in Leinster history to score four tries in a European Champions Cup match, when doing so against Bath during the pool stage. He is Leinster's leading try scorer (five), second in line-breaks (seven), fifth in defenders beaten (10) and fourth in offloads (four) in the tournament to date. He is a superb footballer and if he maintains that form through the rest of the season, he should travel with Ireland to New Zealand with the prospect of winning a first cap.
Trivia: He was part of the Ireland squad that reached the 2016 Under-20 World Junior Championship final, beating New Zealand en route, the first male Irish national side to do so, before losing to England in the final. He excelled at Sevens where he was a valued member of the Ireland squad for two seasons before returning to Leinster. O'Brien, Tadhg Beirne and Munster scrumhalf Rowan Osborne have all played Gaelic football at underage level for Eadestown in Kildare.
Weight: 89kgs (14st)
Connacht appearances: 11
Why he's so important? The former Brumbies player has been a revelation since arriving on an Irish passport – he qualifies through his Cork-born mother – at the start of the season, quickly endearing himself to team-mates and supporters with his prodigious ability. His balance, footwork, speed, vision, lines of running and kicking game make him such a huge threat with the ball. Despite a period on the sideline, he scored six tries in 11 appearances and won his first of four caps for Ireland during the Six Nations win over Wales. His first try in the green jersey was a memorable individual effort straight from a kick-off in the French game.
Trivia: He has a tattoo of his housemate Oisín Dowling on his thigh. Hansen explained recently that he was thinking about getting some new ink to go with previous tattoos. Dowling asked: "Are you going to get my face on you? Hansen responded: "If you want to pay for it, I'll get it. I didn't think much of it but the next day it was booked in. I have the big fella on me [thigh] for the rest of my life. It looks like Buzz Lightyear has had his tonsils out or something, but it is supposed to be Oisín. I think it was about €100."