Reigning European middleweight champion Jason Quigley secured Ireland's first medal at the World Championships with a fine points win over Hungarian fourth seed Zoltan Harsca.
And light-heavyweight Joe Ward joined Quigley in guaranteeing a medal when he won his bout against Russian third seed Nikita Ivanov, who was prevented from continuing after sustaining a cut in the first round.
But heavyweight Tommy McCarthy lost a split decision to Argentina's Yamil Peralta, and Olympic bronze medallist Michael Conlan went down to tough Russian Vladimir Nikitin at bantamweight.
Paddy Barnes lost a scrappy contest on points to Uzbekistan's Jasurbek Latipov.
Double Olympic bronze medallist Barnes, who had moved up from light-flyweight for the first time, suffered a cut in the first round of a bout in which both men were deducted a point for infringements.