Daniel Wiffen finished fourth in the 1500m freestyle at the World Aquatics Championships in Fukuoka, Japan.
Australian Samuel Short hit the front early and swam the first half of the race in under world-record pace, with the Down man a close fourth at that stage.
But they both started to tire and the closing stages turned into a battle between American Olympic champion Bobby Finke and 800m world winner Ahmed Hafnaoui of Tunisia.
Hafnaoui clinched it by just five hundredths of a second in 14 minutes 31.54 seconds, a championship record and only half a second outside the world record.
Short took bronze in 14:37.28, with Wiffen a distant fourth in 14:43.01, just over eight seconds slower than his personal best but his second fastest ever time.
Wiffen said afterwards: "I'm happy with my World Champs so far, a bit of disappointment there going in with the fastest PB and then coming fourth, but I mean that’s just sport. I guess I just have to train harder and come back faster.
"I’m just going to go back training. I’ve got a vengeance now because I want to win that race and I didn’t. I’m just going to keep training – I’ve got the European Under-23s in Dublin so I’m going to focus on that and post some fast times there and then next season just go for that gold in Paris."
Wiffen, who turned 22 earlier this month, set a European record in coming fourth in the 800m freestyle final on Wednesday but the 30-length race was his preferred distance.
He had been pipped into second by Finke in Friday's semi-final and came in as the fifth fastest in the field based on personal bests.
Earlier on Sunday, Ireland's women’s and men’s 4x100m medley relays both finished in 13th position in this morning’s heats, provisionally placing them within the 16 relay spots available for Paris 2024.
Fukuoka is the first opportunity for relays to qualify for the Games, with a second and final opportunity at the Doha 2024 World Championships, where teams can be moved up or down the rankings.
The women’s team of Danielle Hill (backstroke), Mona McSharry (breaststroke), Ellen Walshe (butterfly) and Victoria Catterson (freestyle) set a new Irish record when they combined for a time of 4:01.25. The quartet had held the previous standard of 4:02.93 from the 2021 European Championships.
The men’s team of Conor Ferguson (backstroke), Darragh Greene (breaststroke), Max McCusker (butterfly) and Shane Ryan (freestyle) came home in 3:35.03 to win their heat, just 0.41 outside of the Irish record of 3:34.62 also set in 2021.
