The 44-year-old TV presenter – who rose to fame after winning the second series of Big Brother in 2001 – is married to Armenian professional dancer Arthur Gourounlian, 42, and they became parents to daughter Blake on Thursday, 1 September.
Dowling has now shared a lengthy post thanking his "absolutely incredible" sister, revealing she has endured "vile" and "disgusting" messages from trolls for carrying his child – whose biological mother is an anonymous egg donor.
He said: "You are an absolutely incredible human. From our first jokingly conversations about this back in 2015 when you moved in with us in London to our more serious chats about it in January 2021 in the middle of the pandemic when we would go on our daily walks. This has been in the works for years.
"Your dedication and focus has been out of this world. You have always put yourself out there for everyone to voice their opinions about this and you never faltered. Even the vile/disgusting messages we all received just encouraged us to be more open regarding our surrogacy journey."
Watch: Brian Dowling and husband introduce their baby girl
Dowling revealed there had been struggles to enable Aoife to take hormones to aid fertility due to the lack of surrogacy legislation in Ireland.
He confessed he and husband Gourounlian – who he married in 2015 – had been ready to give up with their struggle to conceive through IVF egg donation, but it was his sister who encouraged them to keep trying.
The former SM:TV Live presenter added: "You aren't just my sister or Arthur's sister-in-law, you are an angel and my best friend. You deserve the world and everything you desire. We love you Aoife darling you are our queen and long may you reign."
The Irish star rose to fame as the second winner of Channel 4 reality show Big Brother and also won Ultimate Big Brother in 2010. He went on to host the show for Channel 5.
Dowling proudly announced on Instagram to his 208,000 followers: "Please be upstanding for the arrival of our beautiful daughter Blake Maria Rose Dowling Gourounlian.
"[She] was delivered safely on Thursday September 1st at 14:52pm weighing 7lb 4ozs.
"We are absolutely, totally with her and can’t believe she’s here and is ours.
"None of this would have been possible without our donor, a woman we have never met or even seen a picture of but has given us the gift of life."
He added that sister Aoife is "a saint to us" and he and his husband "will be grateful for the rest of our lives".
He said: "Baby Blake can’t wait for her Aunty Aoife to spoil her."