The 6ft 7in Byrnes has joined the Irish province from Australian side Eastern Suburbs, and is already armed with an EU passport.
Cheika said: "Adam is a young, hard and aggressive player who will add competition and skills to the pack.
"He played for Eastern Suburbs and he always impressed me when I was coaching Randwick.
"The rivalry between the two sides is like that between Leinster and Munster. So for me to bring him here shows how highly I rate him."
Young winger James Norton has agreed a new one-year deal with the province.
A former Ireland Schools and Under-21 representative, Norton had a stand-out season in 2003/04, scoring eight tries in 13 Celtic League games, but struggled to find first- team form last season.
The move came in a busy week for Leinster Rugby. The province were busy putting the finishing touches to the pitch re-laying of Donnybrook (pictured above) and their new home and away jerseys were launched in Dublin on Friday.
Up north, former Ireland B winger Terry McMaster has been confirmed as the new Ulster under-21 coach with recently-retired Ulster and Ireland lock Gary Longwell to function as his assistant.
Ulster Rugby have also signed a deal with local company Titan Environmental Ltd. to become an official sponsor of the the Ulster squad.
The sponssorship will see the name of the environmental storage tanks company feature above the numbers of the Ulster players' jerseys.
Left without a professional contract, inspirational Shannon captain Tom Hayes, who was at the fulcrum of the Limerick men's drive to the AIB League Division One title back in Mary, has signed a one-year contract with English Division One side Plymouth.
The 25-year-old lock said of his move: "To leave a club like Shannon is really tough, but without a professional contract in Ireland, I felt the time was right to make the move across the water. I travelled over to Plymouth last week and was very impressed with their set-up.
"They have made huge strides in the last five or six years having made their way from the Third Division up to third place in the First Division. There is huge ambition at the club to break into the Premiership."
Shannon, who retained the Division One trophy in coach Mick Galwey's first season in charge, have signed young stars Fergal Walsh and Donnacha Ryan from UCC.
Both players have played previously with the Ireland Under-19s, and lock/flanker Walsh figured in the Irish squad at April's IRB Under-19 World Cup in South Africa.