The singer described Stephen - who died at his Majorca home, in Spain, last weekend aged just 33 - as his "brother and wing man" and vowed to continue the band in his honour during his eulogy at the service held at the St Laurence O'Toole Church in Dublin, Ireland.
A tearful Ronan said: "The world has lost one of its brightest stars. We have lost a brother and I have lost my wing man. For Stephen, we will carry on but the band will never be the same without him.
"Forever young and never forgotten. A friend, a brother, a son, a husband and a hero. We are going to miss you brother, we love you and we always will. We know you have found peace, perfect peace."
Bandmate Mikey Graham added: "He brought colour to the band - and that band now feels black and white without him."
Boyzone - also comprised of Shane Lynch and Keith Duffy - also performed Ronan's solo track 'In This Life' to around 550 mourners inside the church. Lyrics included "Deep in my heart/The only dream that mattered had come true/In this life, I was loved by you."
As well as Stephen's parents, family and civil partner Andy Cowles, other attendees included 'X Factor' judge Louis Walsh - who managed the band for 16 years - Jason Donovan, the members of Westlife, Kian Egan, Shane Filan, Nicky Byrne and Mark Feehily, and ex-Westlife singer Brian McFadden.
As he entered the church, a sombre Louis said: "'It's a difficult day but we have to celebrate his life.
"It's an incredible atmosphere in the church, all his best friends are there, his family are there, it's what he would have wanted."
Speaking about Elton, David said: "He's completely bereft."
Around 3,000 mourners laid tributes outside the church and listened to the service on loudspeakers.
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