Proud parents, Ciara and Mark, watched on at Sydney's Allianz Stadium as their sons Cian and Sam started together for Ireland for the second time in five months.
That first occasion saw Andy Farrell's men fall to a disappointing defeat away to France, but a gutsy team performance, as they came from behind four times against the Wallabies, drove them to a 33-31 bonus point victory.
The Prendergast brothers book-ended the scoring for Ireland, with Cian crossing for the visitors' opening try in the 10th minute, and Sam coolly splitting the posts with a late match-winning conversion.
What proved to be an enthralling attack-dominated encounter could have taken another twist right at the death, but Ben Donaldson missed a long-range penalty, as Ireland prevailed by two points.
Reflecting on a Nations Championship opener that had numerous momentum swings, Cian said: "It was a weird old game. In fairness, Australia were good in the first half. They were physical.
"It didn't feel like ourselves out there in the first half at times, but in fairness, the bench made a huge impact, and the grit of the group. We dug out a win, so delighted with that.
"Defensively we felt like there was just a little bit more in us, and that was just the feeling.
"Obviously we'll look back in the game and see what happened, but, look, there is definitely more in us, and that's the exciting thing as well."
Building on his first two Six Nations games, Prendergast reclaimed the number 6 jersey for the clash with Australia as he won his seventh international cap since last July.
His return to Ireland's starting back row was just reward for a strong finish to the season as Connacht captain, as he helped the province to feature in the BKT United Rugby Championship's knockout stages and qualify for the Investec Champions Cup.
The Kildare man was one of Ireland's leading tacklers against the Wallabies, landing 17 tackles (including one dominant hit), and was also a regular target in the lineout with five takes - one ahead of James Ryan and Tadhg Beirne, the player who replaced him after 51 minutes.
Prendergast admitted: "I was delighted to get the opportunity, and it's brilliant to be playing with lads like Jack (Conan), Josh (van der Flier), and Nick (Timoney), Tadhg (Beirne) as well.
"It's been a great week of build-up and they've been brilliant for me. Just little conversations. I'm just delighted we got the result in the end.
"Obviously since January, the Connacht season turned around and we went on a run.
"Unfortunately, we didn't get the end result we wanted, but it's brilliant to be part of this tour, such an exciting tour, and to get the result today and be part of a famous win like that is brilliant."
Cian was watching on from the sideline when Sam was presented with the opportunity to convert Thomas Clarkson's crucial 77th-minute score, and edge Ireland back in front.
The pair are the latest set of brothers to play together for Ireland at the highest level - the 57th in all - following in the footsteps of the likes of the Wallaces, the Easterbys, the Bests, the Kearneys, and the Scannells.
Defeating the Wallabies on Australian soil is a rare achievement for Ireland, who had only done it on five previous occasions - in Sydney in 1967, and twice each during the 1979 and 2018 tours.
Sam had missed the conversion of Josh van der Flier's try from a similar position, to the left of the posts, during the first half, but Cian had full confidence that his 23-year-old sibiling would turn his last kick into a decisive two-pointer.
"Obviously I'm delighted for Sam to slot the kick," he reflected. "I thought he was brilliant and managed the game excellently in the second half in the right areas of the field.
"He kicked really well, and yeah, some set of stones on him to slot the kick at the end.
"Obviously he had a bit of a tough Six Nations and a tough (period) until the end of the URC season, so I'm really stoked for him with the way the season has turned around.
"He was excellent against the Bulls and he was excellent in that run-in for Leinster. For him to kick that kick at the end, it just shows the character he is."
The Prendergasts combined to score 13 of Ireland's 33 points, in the form of a try and four conversions, and the occasion was made all the more special for them by being able to celebrate with their parents straight afterwards.
"It's class," agreed Cian. "Our parents arrived in on Thursday evening. We were chatting to them after (the match). It was cool.
"Every time you play for Ireland, there's an element of pressure. We don't want to make it about ourselves when we both get picked together.
"It is about producing a performance for Ireland, and I'm just delighted we got the result.
"Obviously the game in Paris (at the start of the Six Nations), the last time we started together, didn't go the way we wanted to, so it was brilliant to get the result here."
