She told the Herald she was "over the moon" to be selected by the judge she admired the most.
"I can't believe it, I'm so shocked. It is just fabulous. Our next stage is the battles and we are working really hard with the songs," she said.
As well as having a famous brother, Niamh's dad Rob Strong played in The Plattermen, a showband in the 1970s.
Niamh sang soul classic Billy Taylor's I Wish I Knew What It Was To Be Free and said she used her real surname because she wanted to be judged on her own singing ability.
"Armstrong is my second name. My dad and brother took 'Strong' as a stage name," she explained.
"I felt going into the show that I should do it on my own merit.
"If I did get through the blind audition, I didn't want people to say that I got through because of name. I didn't tell anyone there, so I don't know how they found out."
Niamh (32) who lives in Naas, Co Kildare, with her son Elliot (2) said that she remembers the hype surrounding the launch of The Commitments.
"There are seven years between Andrew and I. I remember going to the Savoy and watching The Commitments, thinking 'That's my brother up there'," she said.
"I didn't understand it all but as the years went on I realised the importance of it. I started singing with my dad age 11. I grew up with music in the house, there were always musicians jamming."
Niamh said that her brother is still performing regularly around the world and was excited about the idea of a sequel to the book, and movie, which made his name.
"Andrew's doing fantastic, he is in France touring at the moment, I'm delighted for him."
She previously came close to stardom in LA but was too homesick to follow the dream.
"I was represented by two music groups and by Ray Charles's management when I was 18 or 19.
"I had my own band -- The Niamh Strong Band and supported Angie Stone in America but I felt too homesick to stay.
Niamh went to study dental nursing at Trinity College but said that music is her first love, adding: "I have never given up my dream."