ITV2 announced that Niall has left the villa in June after just over a week inside.
According to a statement from ITV, Niall left "for personal reasons".
Now Niall has revealed just why he left the show in a lengthy Instagram post, sharing he has Asperger syndrome.
Niall wrote: "For far too long, I have suffered in silence and not acknowledged a massive fact about my life which going into the villa has led me to finally realise and accept," he wrote.
"When I was a young child, I was diagnosed with Asperger syndrome, a fact that until this post has never [been] shared outside of my close family.
"Growing up was extremely difficult for me, and I often felt out of place. I always felt that people didn't understand me, yet I was afraid to reveal my true scales as I did not want the label or stigma that was attached to it.
"But now I think it is important that I come forward, not only so that I can finally be honest with myself and to those around me, but also so that other individuals in my position can embrace their true colours."
Niall continued: "It's not been an easy ride for me to come to terms with this fact but I am glad that I can now accept who I am, and am looking forward to my next chapter.
"I would just like to thank the team at ITV for always backing me and giving me the opportunity to rid my myself of my insecurities and embrace the fact that I am different yet I am still a rainbow fish. I can't explain how grateful I am for the support from the whole team over this period.
"I would also like to thank the British public for the love you have all shown me over the past few weeks, it has been overwhelming."
He concluded: "Now it's time for this rainbow fish to dive deep into the big blue ocean and show the world what I'm all about - there's more layers to come! 🐠#AutismAwareness #BeYourOwnKindOfRainbowFish #LessPrangMoreLove"
In the villa, the rest of the Islanders were informed of Niall's departure by Alex, who announced: “For personal reasons unfortunately he can’t continue or come back to the Villa.”
Reacting to his departure in the Beach Hut, Niall's partner at the time Georgia said: “Obviously I was coupled up with him so obviously I came closer to him than most people in here but it wasn’t just that. He was actually my friend, my buddy.”
Alex meanwhile added: “I’m obviously sad that Niall’s left the Villa. We got on really, really well. He’s such a great guy.
"And I look forward to catching up with him and having some fun with him outside.”
Love Island airs on ITV2.