In recent seasons Declan experienced a number of concussive episodes. His symptoms were progressively slower to resolve and he was referred to a leading neurologist by the medical team at Ulster Rugby.
Following tests it was recommended that it would be in his best long-term interests to stop playing.
Declan joined Ulster Rugby in the summer of 2006 and made his debut for the province in a 31 points to 16 win over Scarlets in September that year. He scored his first try in his second game for the province, against Newport Gwent Dragons.
Nine seasons on and almost 90 appearances later he scored his third and final try for the province, also against Dragons, in November 2014.
He made his test debut, against New Zealand, in Auckland, in the 2012 summer tour and won his final cap, as a substitute in Ireland's defeat to the All Blacks in the 2013 Autumn Internationals.
Commenting on his decision to retire, Declan said: "While it's not an easy decision to hang up the boots, I can look back with fondness at nine fantastic seasons at Ulster Rugby.
"It was a huge honour to have represented my country and an enormous privilege to pull on that green shirt. I have played alongside some brilliant individuals and have made friendships that will extend long into my retirement from rugby.
"I want to thank the medical teams at Ulster Rugby and the IRFU for the care that they have given me. My symptoms continue to improve and I know the advice that I received is in the best interests of my long-term health.
"Finally, I would like to thank the staff at IRUPA who have been a great support in recent seasons and who have helped me make the transition from playing rugby to my new job as a quantity surveyor."
Ulster's team manager, Bryn Cunningham, hailed Declan's distinguished playing record:
"Declan has been a tremendous servant for both Ulster and Ireland. Having played almost 100 times for the province and seven times for Ireland is evidence of a long and successful rugby career.
"He will be missed both on and off the pitch and I know everyone connected at Ulster Rugby - players, management and supporters, will wish him well in his retirement."
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