Cotter put in a impressive performance in the women’s U23 race in Portugal, holding off the challenge of Jasmijn Bakker of the Netherlands on the final lap.
Denmark’s Anna Emilie Moller comfortable won the race by 40 seconds ahead of Dutch runner Jasmijn Lau.
Aided by strong performances by Omagh twins Eilish and Roisin Flanagan, who finished ninth and 17th, Ireland claimed second in the team event.
Fian Sweeney’s run to 38th place, along with Claire’s Fagan in 47th Sorcha McAllister’ in 57th place, capped a strong day for Ireland’s U23 women.
Brian Fay was the first Irish runner home in the men’s U23 race, finishing in 22nd, while Jodie McCann was 46th in the women’s under-20 race.