Aidan defeated Merven Clair, from Mauritius, in the quarter finals on a split decision to guarantee at least a bronze medal from the games.
The 24-year-old Team Ireland welterweight now faces Team GB’s Pat McCormack in their semi-final on Sunday.
Speaking after his triumph, Aidan told the BBC: “This moment is going to be with me for the rest of my life. Absolutely crazy!”
Thanking his “unbelievable” supporters, he added his target now is “gold”. “Always go for gold”.
Cheering Aidan on was his sister Michaela, also a Monkstown Boxing Club member and Olympian, who made it through to the last 16 in the featherweight division before losing to Italy’s Irma Testa.
Michaela Tweeted: “Me and my brother Aidan have trained together and grown together our whole lives. When me and him qualified for the Olympic Games on the same day, it was the most special day of my life, but watching him win an Olympic medal brings a joy to me I’ve never felt. That’s my baby bro!”
Taoiseach Micheál Martin extended his best wishes too.
“Congratulations Aidan Walsh on a fantastic fight and another guaranteed Olympic medal for #TeamIreland,” the Irish premier said on Twitter.
The Mayor of Antrim and Newtownabbey, Councillor Billy Webb, said: “I was delighted to hear the news this morning that Aidan Walsh has made it through to semi-finals. To secure at least a bronze medal at the Olympic Games is an outstanding achievement and we are incredibly proud of him.
“I wish him every success for Sunday.”
Monkstown Boxing Club summed up the joyous mood with the following Tweet:”Champagne anyone!”