Daniel Wiffen has missed out on a medal in the men's 800m freestyle at the World Aquatics Championships in Singapore after an eighth-place finish in the final, while Ellen Walshe has qualified for the final of the women's 200m butterfly by lowering her own national record.
Olympic champion, Wiffen, who was defending the world title he won in Doha last year, was starting in lane one, and ultimately clocked a time of 7:58.56 as Tunisia's Ahmed Jaouadi (7:36.88) won gold ahead of German duo Sven Schwarz and Lukas Martens.
The 24-year-old led the early stages of the race from the outside lane, before slipping down to fifth midway through and then seventh and eighth at the 500m and 700m marks respectively.
Speaking after the race, Wiffen, who is recovering from appendicitis, took the result in his stride.
"I know last time I spoke, I didn't think I was going to make the final but we got in there, we got eighth place, last in the final," he said.
"Obviously, not my best showing, from last year winning the Olympics to coming eighth in the final. But honestly, I'm happy to be in the final, still progressing.
"Still getting those nerves up. That's what sport's about, trying to get myself comfortable and I was definitely comfortable in that race.
"I'm pretty sure I was leading at one point, and then I got to the 400 and then (my) stomach was in pieces.
"So, not my best showing but I'm happy to be in the final, happy to represent Ireland again in the final."
Wiffen had qualified for the final after setting a time of 7:46.36 to finish fifth in his heat on Tuesday.
That left him with a nerve-wracking wait to see if he would progress, ultimately doing so as eighth fastest overall but leaving him starting in an outside lane for Wednesday's final.
After his heat, the Armagh native had subsequently admitted that he was feeling "really weak at the moment."
He will also race in the 1500m freestyle on Saturday.
Shortly before the final, one of Wiffen's rivals, Australian swimmer Sam Short, skipped the race on medical advice after food poisoning.
The 21-year-old was among the top medal contenders in a stacked field but said he had to take the "extremely emotional" decision as his illness escalated quickly.
"After an awesome heat swim which I felt phenomenal, I got very ill very fast, which we believe is food poisoning from my hotel lunch," Short wrote on Instagram.
Short, who won the silver medal in the 400m freestyle event on the opening night of competition, had qualified for the 800m final with the second-fastest time of seven minutes and 42.22 seconds, behind Jaouadi and ahead of Schwarz.
He was replaced in the race by fellow Australian Benjamin Goedemans, who clocked 7:48.66 in the heats, and finished seventh in the final.
Ellen Walshe was the final Irish swimmer in action on Thursday and qualified as fourth fastest overall for Thursday's women's 200m butterfly final with a new national record.
The 23-year-old Templeogue swimmer finished third in her semi-final heat in a time of 2:07.48, beating the 2:09.15 she had set in the morning heats.
She also broke the 2:08.42 Irish record she had set in style during the Leinster Aspiring Champions meet in March in the process.
Walshe, who was a finalist in the the 200m individual medley earlier this week, had brought the 200m butterfly back into her repertoire for the first time in five years earlier in 2025.
"I'm delighted," said Walshe.
"It was an event I really didn't want to take back up, and I think just to see how far I can get it, I have to be happy. As a junior level I really struggled to kind of move it on, but now I can see a lot of progress and the back ends definitely there."
Walshe will now have just under 24 hours to prepare for Thursday's final which will be at 12.02pm Irish time. She added: "I'm looking forward to tomorrow to see if I can kind of be a little bit better through the front speed a bit, but yeah, I'm in against some really big names, so it's going to be exciting."
Earlier, Danielle Hill missed out on a place in the final of the women's 50m backstroke after finishing 12th fastest overall in the semi-finals.
The Larne swimmer was seventh in her semi-final heat in a time of 27.71, which was quicker than the season's best 27.84 she posted in this morning's qualifying heats, and just 0.07 shy of the two-time Olympian's national long course record in the event.
But it wasn't enough to make it into the top eight that advance to the final.
"Very mixed emotions," Hill said.
"Looking at it, it is fast. It's now become an Olympic event, so I suppose a few more people are a little bit more switched on.
"Of course I want to be in that final, but I've been around that time now for a couple of years, so there's something there that we've got to look at and change in order to get that improvement.
"So, all in all disappointed obviously not to make that final, but massive learning, and last week I wasn't even swimming 25 metres of backstroke and I've done two races now so I'm happy."
Evan Bailey missed out on a place in the 100m freestyle semi-finals but came close to his personal best in finishing in a time of 49.52 on the way to a second place finish in his heat.
It is the Limerick swimmer's final individual event of the championships.
"It was a decent swim, it wasn't anything special, it was 0.4 off my PB, but yeah, just to get in, swim, get that experience here, get it under the belt, so next time I come back, I'll know exactly what to do," he said.
"Not the best of my life, but the focus hasn't really been sprint and the 100 free recently. The last few months I’ve been up at altitude and working more on my endurance and just build my aerobic capacity to build that 200 (freestyle), so I’m not overly disappointed with that time for the 100 as it wasn't really the main focus, but hopefully now I'm going to try building the speed in the next coming months and I'll get that 100 better along with the 200, so I’ll have multiple events."
Meanwhile, France's Leon Marchand, who won four gold medals and a bronze on home soil at the Paris 2024 Olympics, set a new world record in the semi-final of the men's 200m individual medley, breaking Ryan Lochte's old time by 1.3 seconds.
You can watch the action on Eurovision Sport
