Swimmer Shane Ryan finished last in his 100m backstroke semi-final in the early hours of Monday morning.
Ryan, who swam a new personal best of 53.85 in his heat, would have needed to take a considerable chunk off that time in order to reach the final.
The American-born swimmer made a good start in lane one, and was sixth at the turn.
However, he faded on the home straight and finished eighth in the heat - and 16th overall - in a time of 54.40.
His semi-final was won by the USA's Ryan Murphy in 52.49.
As well as swimming a personal best, Ryan had set a new Irish record of 53.85 seconds to reach the semi-finals.
That time was just one hundredth of a second slower than his best ever time of 53.84, set at the US Nationals in 2013.
The Irish record had stood at 54.21 from the European Championships in May of this year.
Ireland’s last semi-finalist in swimming at the Olympic Games was Andrew Bree in Beijing in 2008 when he made the 200m breaststroke semi-final.
22-year-old Ryan returns to the pool for the 100m freestyle on Tuesday.
Elsewhere it was a good day for the Irish boxing team after a difficult week with events outside the ring.
Steven Donnelly produced a comprehensive performance to get through to the last 16 of the 69kg Welter division after he dispatched Algeria’s Zohir Kedache by unanimous decision.
The 27-year-old was dominant from the off and requires two more victories to be guaranteed a place on the podium.
Donnelly will be back in action on Thursday to take on Mongolian Tuvshinbat Byamba in the last 16.
In three-day eventing, Jonty Evans scored 41.80 aboard Cooley Rorkes Drift and Mark Kyle (Jemilla) posted a score of 50.40 in the dressage.
They are in fifth place as they look to challenge the leaders in Monday's crucial Cross Country phase - traditionally a strong point for Irish teams..
The individual and team Eventing medals will be decided following the final Show Jumping phase on Tuesday.
Back at the Aquatic Stadium, there was disappointment for Fiona Doyle in the 100m breaststroke.
The 24-year-old found herself in a very competitive heat and a disappointing finish saw her finish in eighth position with a time of 1:07.58.
She finished 0.26 seconds and four places outside the top-16 that move on to the semi-finals in that particular event.
There was no rowing action today as weather conditions played havoc across the Lagoa.
