Replacements used: Ulupano Seuteni (QLD) for Burton (2-12 mins, blood sub), Silatolu Latu (NSW) for Paraka (half-time), Jack Dempsey (NSW) for Baldwin (44), Allan Alaalatoa (NSW) for Hoskins, Reece Hodge (NSW) for McIntyre, Ben Meehan (QLD) for Wessels (all 62), Patrick Sio (NSW) for Toleafoa (76), Phil Kite (QLD) for Alaalatoa (77), Seuteni for Burton (79).
Replacements used: Sean McCarthy (Shannon/Munster) for Thornbury (7 mins), Ryan Murphy (Dolphin/Munster) for Joyce (58), Peadar Timmins (UCD/Leinster) for Donnan, George McGuigan (Newcastle Falcons/Exile) for B Byrne (both 65), Adam Boland (Lansdowne/Connacht) for Taylor (67), David Shanahan (Clontarf/Leinster) for McGrath (72), Ryan Furniss (Worcester Warriors/Exile) for E Byrne (77), David Panter (Corinthians/Connacht) for Wootton (78).
A 20-0 deficit proved too much for Ireland to overhaul, as Mike Ruddock's charges paid the price for a frustrating first half facing into a strong wind. They leaked two converted tries and two penalties in the opening 25 minutes.
A late penalty from centre Rory Scannell opened the Irish account before the break and they stormed out of the blocks on the restart, with Scannell landing a second penalty and full-back Rory Scholes breaking through for an unconverted try.
A Luke Burton penalty was cancelled out by two more successful strikes from Scannell as Ireland, 19-15 winners over Australia in the pool stages, cut the gap to 23-17 with their powerful scrum exerting more influence.
However, the Junior Wallabies gave themselves some crucial breathing space with their third try of the game - a breakaway score by Alex Northam on the hour mark - and Ireland were unable to respond in an energy-sapping final quarter.
The early exchanges in La Roche-sur-Yon were particularly physical as the Australians looked to gain revenge for that defeat on the opening night of the tournament.
A bout of pressure yielded a penalty and out-half Jake McIntyre kicked Australia ahead in the fourth minute. Scannell was presented with a long range opportunity to reply but he failed to split the posts.
A couple of offside decisions counted against Ireland and with the wind-backed Junior Wallabies continuing to force the issue, a well-struck penalty from centre Burton made it 6-0.
Ireland had been forced into a late change as Adam Byrne cried off through injury and Mark Roche, more noted as a centre, took his place on the right wing.
The Australians then profited from a turnover in the Irish half and worked the ball to the left for Harry Parker to break a tackle and run in a converted try.
Flanker Michael Wells added another just four minutes later, going over the top of the close-in ruck to score after prop Oliver Hoskins had been held up just short.
Now 20 points adrift, Ireland had a huge amount of ground to make up. They made improvements approaching half-time with half-backs Luke McGrath and Steve Crosbie distributing well behind a hard-working pack.
The fruits of their labour came in the form of a 36th minute kick from the left-footed Scannell, who clipped over a simple penalty after a strong carry from prop Christopher Taylor.
Ruddock's half-time words obviously worked a treat as Ireland made great headway on the restart with eight points in the opening two minutes.
Ulster prospect Scholes showed his fine finishing skills again as he ran in his fourth try in five games in France. Scannell, who landed a penalty just moments earlier, was off target with his conversion attempt.
The momentum was with Ireland and although Burton clawed back three points for the Australians, the Irish forwards used their highly regarded scrum to keep them on the comeback trail.
Scannell kicked two of three successive penalty attempts to put only six points between the sides and suddenly the game was there for the taking.
But Australia had looked more dangerous with ball in hand though and Ireland lacked their usual accuracy in the tackle area. A long pass out to the left from replacement Reece Hodge set up winger Northam's 63rd-minute try in the corner.
It was the decisive score as the Ireland youngsters ran out of steam in the closing stages against a committed and well-organised Australian defence.
It was a disappointing note for Ireland to finish a tournament that had held so much promise for them - from the historic opening victory over Australia and runaway triumph against Fiji to the heroic comeback against New Zealand that had them close to reaching the semi-finals for the first time.
Hampered by a couple of notable injuries, they tasted defeat twice in the knockout stages - losing 9-8 to hosts France on Tuesday night - but Ruddock believes that this group of players can build on what they have achieved this season.
"We picked up a few injuries, people like Tom Daly who was particularly crucial to us. We missed guys like that in the second week that could have made a difference. Adam Byrne dropped out today as well," he said.
"We don't hide behind the injuries, we just tailed off a little bit in the second week. But I'm still proud of the effort, the first week in particular I thought we played some really good rugby.
"We've had wins over teams like South Africa, England and France at this age grade and in this tournament we beat Australia. I think Irish rugby is continuing to progress nicely on the world stage."
The Welsh native added: "There's some good players in the Irish camp and I think a lot of them will go on to bigger and better things, particularly if they can play to the standard they showed in the first week.
"There's a lot of good players who will go on to become professionals there, they're a good outfit overall. Some players grew and got better and better throughout the tournament.
"Guys who were originally second choice have come through as well and really put pressure on the guys who had the Six Nations berths. We've had a few guys who gained ground on the other guys.
"I think the pack, in particular, have grown a good reputation in the tournament. Our scrummaging has probably been as good as any team in the tournament. So, we'll take a lot out of the tournament even though we were disappointed with our second week."
TIME LINE: 4 minutes - Australia penalty: Jake McIntyre - 3-0; 9 mins - Ireland penalty: missed by Rory Scannell - 3-0; 13 mins - Australia penalty: Luke Burton - 6-0; 20 mins - Australia try: Harry Parker - 11-0; conversion: Luke Burton - 13-0; 24 mins - Australia try: Michael Wells - 18-0; conversion: Luke Burton - 20-0; 36 mins - Ireland penalty: Rory Scannell - 20-3; Half-time - Australia 20 Ireland 3; 41 mins - Ireland penalty: Rory Scannell - 20-6; 42 mins - Ireland try: Rory Scholes - 20-11; conversion: missed by Rory Scannell - 20-11; 52 mins - Australia penalty: Luke Burton - 23-11; 56 mins - Ireland penalty: Rory Scannell - 23-14; 59 mins - Ireland penalty: missed by Rory Scannell - 23-14; 61 mins - Ireland penalty: Rory Scannell - 23-17; 63 mins - Australia try: Alex Northam - 28-17; conversion: missed by Luke Burton - 28-17; Full-time - Australia 28 Ireland 17
Referee: Marius Mitrea (Italy)