Ronan Keating, Shane Lynch, Mikey Graham and Keith Duffy admitted they were unsure if they would carry on as a four piece after Stephen died of natural causes at his holiday home in Majorca in 2009 but eventually decided to make the 'Brother' album and go on tour to honour their friend.
Speaking on stage, Ronan told the crowd: "12 months ago we didn't know what the future held for us, we didn't know if we could tour as a four piece. We made this album under unbelievable duress, we laughed, we cried.
"I hope you're ok Steo and I hope you're enjoying the music."
Mikey added: "I try not to think too hard about the loss, He's never forgotten. If I could say something about him it's that it's difficult without you."
The group then performed 'Gave It All Away' as a montage of Stephen played on big screens behind them.
In another nod to their dead friend, all wore a touch of red, Stephen's favourite colour as they performed 'Better' and asked the audience to sing his part from 'No Matter What' because he is "irreplaceable".
In a two-hour set the band mixed new songs from their 'Brother' album including 'Til The Sun Goes Down' and 'Ruby' with old favourites 'Picture Of You', 'You Needed Me', 'Love Me For A Reason', ' When The Going Gets Tough' and their first UK number one 'Words'.
As they reappeared in green velvet suits for the encore fans were treated to 'Baby Can I Hold You' before they ended the show with Ronan's solo single 'Life Is A Roller Coaster'.
Support on the tour comes from girl bands Wonderland and Barbarellas, and Australian pop star Guy Sebastian.