The 31-year-old died on Monday having been in a critical condition for over a week, after he was found unresponsive in a Dublin prison cell.
He was serving a two-year sentence after being caught with 446 ecstasy tablets. He previously worked as a fitness model.
Shannen, who appeared on last year’s ‘Love Island’ as one of the Casa Amour bombshells, said she was “completely and utterly broken”, as she paid tribute to Daniel.
Writing on Instagram, she said: “I love you so much. No one will ever understand how much, and I know deep down I’ll never love again. In life we all have that one and you were my one.
“What’s killing me most is we finally were happy, not in the best of situations - you in there, me out here - but we were making it work and we got back on track after everything we put each other through.
“All we talked about was New Years in Thailand (how you were going to put a monkey in a Lidl bag and bring it home with you, your words exactly) and how 2019 was our year - a fresh start - and how you knew you loved me and wanted us.
“I never ever stopped wanting you since the day we met the 19th of January 2015. We had this talk only a few weeks ago and we both agreed we loved each other too much, we were so protective about each other and so jealous. We just done stupid things, aired it for everyone to see and hurt each other, but in the end we came back to each other and that’s all that matters.”
She continued: “I am completely and utterly broken. I’ll never be the same Shannen again and I don’t know what I’m going to do to cope with loosing you.
“Since the day this happened all I could think about was the last phone call, 6 o’clock on the Thursday, and your words were you really do everything for me. I love you to bits. Them words will never ever leave my head. I’m still wishing this is all a bad dream and when I wake up we can start the life we planned for when you got out and just be happy.”
Shannen has since deleted her Instagram account.
Father-of-one Daniel was sent to prison in February, after being charged on drugs offences in November 2014.
Another inmate in Dublin’s Mountjoy prison was also rushed for medical treatment on the same day Daniel was found in his cell, and an investigation into the two incidents is currently underway.