Police said they believed the 18-year-old from Creggan was “most likely still alive when the blaze started”.
His body was found in the stolen red Mazda 6 car in Fairview Road on Saturday morning, after it had crashed into a lamp-post.
His remains have been released to his family in a closed coffin.
Caoimhin’s funeral will take place this Friday, June 7.
Requiem Mass will be celebrated at St Mary’s Church in Creggan at 12 noon.
Interment afterwards in the City Cemetery.
A family notice reads: “CASSIDY-CROSSAN, 1st. June 2019. Suddenly, Caoimhin, beloved son of Barry and Christina, loving brother to Caitlin, Darryl, Brandon, Lennon and Grace, dear grandson of Margaret and the late Daniel Cassidy, and Jimbo and Nora Crossan, dear nephew of Grainne, Danielle, Sinead and Gary.
“Dearly loved and will be sadly missed by the entire family circle.
“Sacred Heart of Jesus have mercy on his soul.
“Our Lady of Lourdes intercede for him.”
Charles ‘Nucker’ Tierney, Caoimhin’s great uncle, has called on witnesses to “help this poor family out”.
“It’s very important that they find out what happened exactly.“I’m appealing for any young lad or girl that was in the car, or witnesses, to come forward.
“Go to the police, a local priest, an activist or me with information,” Mr Tierney said.
A post-mortem examination revealed Mr Cassidy had not been seriously injured in the crash and died after the car caught fire.
Police have said they want to speak to anyone who may have been with the teenager when he died.
They said the car, a Mazda 6, was stolen from a house in Oakfield Crescent earlier on Saturday.
Image caption
Charles Tierney is Caoimhin Cassidy’s great uncle
“Although Caoimhin was in bother a lot he was quiet too,” his great uncle told BBC Radio Foyle.
“People have been telling me how he was looking so well over the past couple of weeks.
“I used to stop him to ask him how he was getting on. I used to help him with his problems and he always thanked me.
“As far as we know his family saw him last Thursday. He would never say if he was going somewhere.
“He would just come in, get changed and head out the door again.
“He was his granny’s blue eyed boy. He lived with her. She spoiled him.
“She and the rest of his family are devastated.”
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