Phil Healy ran a season's best time of 23.21 seconds in the women's 200 metres heats as she missed out on a place in the semi-finals.
Healy finished fifth of five runners in a heat won by Crystal Emmanuel of Canada in a time of 22.74.
Britain's Beth Dobbin finished second, while Jamacia's Elaine Thompson-Herah was third.
Thompson-Herah was crowned 100 metres champion earlier in the Games and was comfortably ahead of Imke Vervaet (Belgium) and Healy.
The versatile Irish runner will return to action on Tuesday when she competes in the 400 metres in what has been a hectic week of racing.
"I'm disappointed that it isn't quicker but I gave it my all," the Cork woman told RTÉ Sport.
"I came out twice already for the 4 x 400 (mixed relay) and I obviously had to sacrifice my individual a little bit for that. But we came away with an Olympic final.
"It is a really short turnaround but still, the Belgian (Vervaet) did it twice and she was ahead of me today.
"Of course I wanted to go quicker but it wasn't there. I feel I was below par but I have another opportunity to come out tomorrow.
"To come out and deliver just off my PB (22.99 seconds) I have to be happy but initially of course I'm going to be disappointed.
"Being the first Irish person in three events, that's making history, the relay made history, so there is so much to take from it but when you give so much to an Olympic year you just want to go out and perform to your best."