Ian MacGeechan's side were expected to steam roll the invitational side but the home side had other ideas. With only five players with Super 14 experience, the Royal XV played way above expectations and with 10 minutes remaining were 25-13 ahead.
But a famous win was snatched away from them when the Lions finally awoke from their slumber to score three tries in rapid-fire succession to secure a shakey victory.
The impressive Byrne took matters into his own hands to start the fightback. He chipped over the defence and when the ball went loose, he showed the footballing skills of Ronaldo to dribble ahead and collect the ball to race over. With replacement Alun Wyn Jones pouncing and Ronan O'Gara racing over virtually on the fulltime hooter, an embarrassing defeat was averted.
The Lions opening performance would not have worried the South African camp with only Byrne and fellow Welshman Jamie Roberts staking a claim for a first Test spot.
The Lions were particularly scrappy in the first half and despite having a superior pack their scrum and line-out malfunctioned.
Naas Olivier and O'Gara exchanged penalties before home skipper Wilhelm Kock, received an inside pass from Sarel Pretorius to pile his way over. Olivier converted then added a penalty as the Lions struggled to stamp their class on proceedings. Hooker Rayno Barnes then compounded the Lions woes when he burst from a ruck and cantered to the try-line.
The Lions managed to put a couple of phases together just before the break when O'Gara fed an inside ball to fellow Irishman Tommy Bowe who scorched over unopposed. With O'Gara adding the conversion, the Lions were trailing 18-10 at the break.
The home side continued to ignore the script when prop Bees Roux took a pop pass to go over for a third try. At 25-13 it looked bleak for the tourists, but Byrne's intervention began a purple patch for the tourists who will have to improve markedly if they are to have serious ambitions of winning the three Test series against world champions South Africa.
Scorers: Royal XV: Tries: Koch, Barnes, Roux; Cons: Olivier, Viljoen; Pens: Olivier (2)
Lions: Tries: Bowe, Byrne, Alun-Wyn Jones, O'Gara; Cons: O'Gara (4); Pens: O'Gara (3)