Meanwhile, John Cooney, whose terms have been agreed for the full 2014/15 season, will continue his shoulder rehab with Leinster during the summer months before departing for Galway.
This is the second instance of 'player loaning' from Leinster to Connacht following flanker Conor Gilsenan's recent three-month stint at the Sportsground where the 21-year-old earned three PRO12 caps (two starts) in the back row.
Gilsenan signed off with his first senior try against the Ospreys, before his departure to London Irish.
Galway-bound Cooney has made a name for himself in the Leinster and Ireland underage and Emerging Ireland set-up as a highly intelligent scrum half.
The former Gonzaga College schoolboy, who plays his club rugby at Lansdowne, made his debut with the senior Leinster team in September 2011 when he featured against the Ospreys in Swansea.
He has since gone on to earn 27 senior caps as well as playing a significant role in the success of the Leinster 'A' British & Irish Cup squads. Cooney has represented Ireland at Under-18 and Under-20 levels and also played three games with Emerging Ireland in the Tbilisi Cup last summer.
Also joining up with Connacht next season is 23-year-old lock Roux. The 120kg South African has played two seasons with Leinster having joined from Western Province in 2012. He also made four Super Rugby appearances for the Stormers before departing for Dublin.
Roux has played 21 times with Leinster in the PRO12 and European competitions, while also featuring for Leinster 'A' in the British & Irish Cup, most recently in their 44-17 final win over Leeds Carnegie which secured back-to-back cup titles.
Roux's first season at Leinster suffered a setback when he was forced to undergo shoulder surgery which ruled him out of action for several months.
Despite serious competition for his position, he notched up ten appearances in year two, including a start in the second row for the RaboDirect PRO12 semi-final against Ulster at the RDS.
Speaking about the new deal with Leinster Rugby and the IRFU, Connacht head coach Pat Lam said: "This is brilliant news for everyone involved. We're delighted to get the loan deal over the line and again, I'd like to thank the IRFU and Leinster Rugby, especially Matt O'Connor and Mick Dawson, for their support.
"Our squad is constantly evolving and there will be plenty of new faces at the start of the pre-season. Kieran (Marmion), who earned his first Ireland cap at the weekend, has had a heavy load the last two years and there is a need to manage his game-time a little more stringently.
"With the scrum half position, John (Cooney) will be pushing for a place along with the incoming Ian Porter as well as Caolin Blade from the Academy so it's great for competition.
"With the unfortunate retirement of Craig Clarke, it certainly helps to have Quinn (Roux) coming in to bolster that area and offer us more selection options.
"We've had glimpses of what these young players can do and they are both promising additions to our squad. They are coming from a highly successful team where there is massive strength in depth and huge competition in their positions so this will be a great opportunity for them to push for more PRO12 and European rugby game-time.
"They are ambitious, hard-working men and hopefully they can make the most of their time here. We're certainly looking forward to welcoming them in to our team."
IRFU Chief Executive Philip Browne added: "The IRFU are delighted that this loan arrangement has been agreed as it can only be a good thing in terms of player development and should provide an opportunity for more game-time for both John and Quinn.
"The IRFU are proud to support four strong and developing provinces and believe that the continued development of players within the Irish rugby family will benefit players, provincial and international rugby in the long-term."