Hailing from Portlaoise where he attended the CBS, Masterson represented the Ireland Under-20s in the 2013 Six Nations game against Scotland.
While his mother is from Westport, it is his Scottish father that enabled Masterson to also represent Scotland U-20s in last summer's IRB Junior World Championship. The young back rower impressed the Connacht coaches in last weekend's 'A' interprovincial clash with Ulster Ravens.
This weekend's match against the Scarlets is Michael Swift's 250th for Connacht. His second row partner Craig Clarke captains the side on Saturday.
In the back-line, Galway man Eoin Griffin makes a welcome return to the centre, while Darragh Leader moves to the right wing. Craig Ronaldson has been selected at out-half.
Commenting on the team selection and the visit of the Scarlets, Pat Lam said: "Eoghan Masterson arrived here during the summer on trial with the Academy and since then, the coaching staff have been so impressed with his professionalism both on and off the field.
"He has worked hard on his game and has truly earned his opportunity and first senior Rabo game. I believe game-time at this level is an essential part of a young player's development once they do the hard work and it's great to see players like Eoghan, Darragh (Leader), Jake (Heenan), Craig (Ronaldson) and Matt (Healy) all following in the footsteps of Robbie (Henshaw) and Kieran (Marmion).
"They had a good win against Ulster the last time out and they've been consistently playing at the top end of the table. Like all our games against Welsh sides, it's sure to be a physical encounter.
"Training this week has been good despite the disruption of losing Danie Poolman to injury on Thursday and only getting Eoin McKeon back training late in the week.
"But there was a great buzz at training and everyone is raring to go. We've prepared well and are looking forward to this fixture.
"We need to keep improving. In the last four or five games we have pushed teams really close. We have shown a lot of heart and commitment, and also created enough opportunities to win all of those games but we were not clinical enough.
"However, there is a real doggedness and determination to get off the bottom of the table and we want to start climbing, commencing tomorrow against the Scarlets."
CONNACHT (v Scarlets): Gavin Duffy; Darragh Leader, Eoin Griffin, David McSharry, Matt Healy; Craig Ronaldson, Kieran Marmion; Brett Wilkinson, Sean Henry, Nathan White, Michael Swift, Craig Clarke (capt), Aly Muldowney, Jake Heenan, Eoghan Masterson.
Replacements: Jason Harris-Wright, Ronan Loughney, Rodney Ah You, Michael Kearney, George Naoupu, Frank Murphy, Dan Parks, Fionn Carr.