The actress, who plays Sash Bishop in the RTÉ soap, welcomed her son with fiancé Craig Hopkins.
Taking to her private Instagram account, she shared an adorable snap with baby Finn, and his big sister Rosie who was born to the couple in 2019.
"Our Finn. One month old today," she said, revealing that he was actually born in February.
The pair revealed last December that they were set to welcome another child.
"This year has been really good," she said announcing her pregnancy in 2021.
"We found out we are expecting another baby, which is wonderful," she told the RTÉ Guide.
"I have to pinch myself that I'll soon be a mammy of three," she added.
Stephanie also has a son named Zach from a previous relationship.
Stephanie and Craig are due to wed in Co Meath later on this year following a romantic engagement on Brittas Bay beach in Co. Wicklow after 3 and a half years together.
The actress was absent from her on-screen role back in November following a brief illness.
Kelly’s character was played by Charleigh Bailey during the week she was unwell.
Taking to Instagram, she gave an update to fans.
"Hey Guys! I've had lots of messages of concern and some questions so just wanted to say I am perfectly fine, I was very sick a few weeks back so had to opt out of filming Fair City for a few episodes,” she said.
"I am already back filming and will be back on screen in the next couple of weeks."
Thanking her understudy for taking over the role, she added: "Also a big well done to Charleigh Bailey who is doing a fab job playing Sash in these episodes!"
Her character has now exited the show for the foreseeable as she takes maternity leave.
The show saw Sash leaving Dublin to be with her close friend Cristiano in Brussels.