Hordur Bjorgvin Magnusson's brilliant free-kick earned Iceland a 1-0 friendly win in Dublin on Tuesday, inflicting a rare home defeat on Republic of Ireland.
Ireland were unbeaten in their last 15 international matches at the Aviva Stadium but struggled to break the well-organised visitors down in a game lacking clear-cut chances for either side.
Bristol City defender Magnusson gave Iceland the lead midway through the first half after a tepid opening, his free-kick clearing the wall and picking out the bottom corner to score his first international goal.
Robbie Brady wore the captain's armband for the hosts after regular skipper Seamus Coleman suffered a broken leg in Friday's 0-0 Group D draw against Wales, but a much-changed Ireland struggled for cohesion.
The hosts did not manage to force any saves from visiting goalkeeper Ogmundur Kristinsson, although four debutants made it a learning experience for Ireland, while Iceland's determined defence held firm to collect their fourth win in five internationals away from home.
With both teams second in their qualifying groups, they will remain hopeful of making it to the 2018 World Cup in Russia, but Iceland look the more likely to reach the tournament on this evidence.
Ireland coach Martin O'Neill handed first international appearances to Conor Hourihane and John Egan, with the latter debutant caught by a stray elbow in the second minute and needing a bandage for his bleeding face.
Brentford centre-back Egan was then booked for bundling over Kjartan Finnbogason after 20 minutes and Iceland scored from the resulting free-kick with the first shot of the match.
Magnussen's strike was floated gloriously over the wall and the ball dipped into Keiren Westwood's bottom-left corner, giving the unsighted goalkeeper no chance of making the save.
Aron Gunnarsson and Finnbogason hit efforts off target as the Euro 2016 quarter-finalists, who had failed to win in seven previous games against Ireland, sought to extend their advantage.
Kevin Doyle glanced a header wide from James McClean's cross after 36 minutes but an experimental Ireland side were unable to muster any shots on target in the first period.
O'Neill's men improved after the interval and Iceland had to clear a string of dangerous crosses before Gunnarsson sent a dipping volley narrowly over the crossbar.
A string of substitutions from O'Neill and Iceland counterpart Heimir Hallgrimsson led to a stop-start second half, with Preston North End pair Daryl Horgan and Andrew Boyle earning debuts.
Substitute Callum O'Dowda had a shot blocked by Sverrir Ingi Ingason after 76 minutes but the equaliser would not come for Ireland, whose long unbeaten run at home is over ahead of a June qualifier against Austria, while Iceland next face Croatia.