Thomas Barr and Mark English were eliminated from their respective events on the second day of action at the World Athletics Championships in Qatar.
Rio Olympics star Barr was fourth in his 400m hurdles semi-final, improving on his heat time of 49.41 seconds with a season's best effort of 49.02.
However, only the top two in each heat qualified automatically and Barr was 10th fastest overall, .09 seconds behind Estonia's Rasmus Magi, who took the eighth and last spot in the final.
Barr's heat was won by American Rai Benjamin in a time of 48.52, while reigning world champion Karsten Warholm set the fastest qualifying time of 48.28.
"I hadn't realised it was .1 of a second, I definitely could have found that in that last hurdle where I messed up," Barr told RTÉ Sport's David Gillick.
"I thought I was going to get 14 strides into it. I wasn't able to drive off it and get that normal burst of speed off it that I normally do. That's what cost me.
"The legs didn't have it in the home straight the same way as I did yesterday, I wasn't as confident in that final kick.
"I am disappointed but at the same time, I know I left everything on the track.
"Such is racing. I've been marginally lucky up until now. Championships have often gone my way, this one just didn't unfortunately.
"I'm looking forward to next year already."
Earlier on Saturday afternoon, Mark English bowed out in the heats of the 800m.
The Donegal native started strongly and took the lead going into the bell, but looked to be in discomfort in the final lap and was quickly reeled in.
The multiple European medallist finished last of the seven runners in one minute 47.25 seconds, the heat being won by Kenya's Ferguson Cheruiyot Rotich in a time of 1:45.98.
"I'm obviously disappointed, I would have liked to qualify for the semi-final," English told RTÉ Sport.
"I knew there were three good guys in that race and it turned out there were six in the end.
"It was either take it by the scruff of the neck or leave it up to hope and for me that was the better option.
"I don't what's been hitting me at 500 metres every time this season, but something seems to be clipping me."
English admitted that his late call-up to the championships had made preparations "difficult", but insisted: "I've made my point about that so I'm just going to move on."
The 26-year-old has just completed his medical studies at UCD and now intends to take a year out to focus on qualifying for Tokyo.
"I'm going to take next year off to go full-time training. 2020 was always my biggest goal when I took that time off. I'd love to make my second Olympic games and give that a good rattle when I get out there."
Brendan Boyce goes tonight in the 50km walk and then the next Irish team member in action is Phil Healy in Monday's 200m heats.