Jared Payne has been forced to retire from his rugby playing career due to a head injury sustained while playing for the Lions.
The New Zealand-native turned Ireland star will remain with Ulster, though, in his new role as defence coach.
He described his retirement as having to "give up the dream".
The 32-year-old Payne ends his career with 20 Ireland caps, four tries and a Six Nations title to his name. He scored a crucial try in the final-day victory over Scotland which sealed that 2015 success.
He added Lions honours last summer, playing three of the warm-up games and scoring a try minutes before leaving the field with concussion against the Chiefs.
Payne has suffered with headaches after training since the blow, which came months after his return from a fractured kidney.
Those symptoms haven't cleared up with a year's rest and rehabilitation.
"It’s been a good ride but unfortunately every good thing has to come to an end," said Payne.
"Playing rugby has taken me to places I never thought I’d see and allowed me meet people I never thought I’d meet.
"I would like to thank all of the medical professionals who have supported me since my injury occurred back in June last year. I’m extremely grateful for your considerable care and attention.
"While I will undoubtedly miss the buzz of running out with mates every weekend, I have to listen to the medical advice and unfortunately give up the dream.
"However, I’ve surprised myself how much I have enjoyed coaching and I’m really looking forward to getting my teeth into this role on a permanent basis."
Payne scored 21 tries in 78 Ulster appearances, and was helping out departing head coach Jono Gibbes in recent months. He will remain involved when Dan McFarland takes over.
He added: "Firstly, I’d like to thank my parents for all their help in my younger years and my brother Josh for being a live tackle bag!
"Thanks also to the coaches and team mates throughout the years that have made living this dream possible, and to the staff, volunteers and fans that make game days so special.
"The support that I’ve received from my partner Chrissie and sons Jake and Tyler, particularly over the past 12 months, has been incredible.
"Finally, I’d like to thank the management here at Ulster for giving me the opportunity to contribute in this way and I look forward to trying to repay the faith shown in me."