George Donaldson, one of the principal singers in the popular group Celtic Thunder, suddenly died earlier this week. The 46-year-old passed away from a massive heart attack.

George Donaldson first joined Celtic Thunder in 2007. The only Scotsman of the group, Donaldson appeared on all nine concert specials and each of Celtic Thunder’s albums.

 Before his death, George Donaldson also had a successful solo career, releasing his first album, The White Rose, in 2011 and his second, The World In My Mind, in 2012. He also won Irish Music Awards in each of those years.

Watch one of George Donaldson perform one of his solo songs, "Glasgow."

George Donaldson’s death came just weeks before Celtic Thunder’s planned tour of Australia. The group pledged to perform the concerts as scheduled in honor of the late singer.

“Donaldson was preparing to join his Celtic Thunder brothers to tour Australia in May, something he always looked forward to,’’ read a statement from Celtic Thunder. ‘‘In recognition of this, and as George would have wanted, Celtic Thunder will respect his wishes and will honour their commitment to tour Australia on the Mythology tour which will commence in May this year.”

Many of Celtic Thunder’s current and former members also paid tribute to George Donaldson after his death.

"To a dear friend, my days spent with you have been some of the happiest days of my life,” said fellow principal singer Neil Byrne. “Always remembering the countless inspiring stage performances we shared and, more importantly, the confidences and laughs. Holding you close to my heart for the rest of my days, my friend George.”

Wach George Donaldson perform "The Old Man" as part of Celtic Thunder.

Glee fans may recognize Celtic Thunder as the group that began Damian McGinty’s singing career. McGinty performed with Celtic Thunder for five years before becoming a finalist on the reality series Glee Project. McGinty won the contest, earning a guest-starring role during Season 3 of the popular Fox series.

Watch George Donaldson, Damian McGinty sing "Take Me Home" with the rest of Celtic Thunder.

Damian McGinty also paid tribute to George Donaldson following the latter singer’s death.

“There are no words that can express my Sadness and Sorrow at the shock passing of a great friend George Donaldson,” McGinty wrote on his Facebook page. "I Shared some of my greatest life moments with him. Those 5 years in Celtic Thunder I will never forget. Im gonna miss you bro. Thankyou for everything. God Bless. X.”

After George Donaldson’s death, he leaves behind his wife, Carolyn, and his 13-year-old daughter, Sarah. Fellow Celtic Thunder principal singer Ryan Kelly mentioned these two loves of Donaldson’s life in his tribute to the late singer.

“Today I lost a colleague, a Celtic Thunder original brother, but, more importantly, I lost a very good friend,” Kelly said. “From the first day I met him at the audition for Celtic Thunder in 2007, we became 'thick as thieves' and shared so many great times together — on and off the stage. I know he's looking down now on the two most important people in his life — Carolyn and Sarah — and the rest of us holding them up for him, with his guitar strapped around his neck and a pint in his hand with the big smile of his. As he always sang Phil's words, in his own inimitable way: ‘though he may be gone, memories linger on, and I'll miss him...My Old "Bro’...Rest in peace Big Man. Those of us in what you deemed ‘Celtic Thunderland’ will miss you more than you'll ever know...”