The bombs detonated at about 5.20am (0320 GMT) as the Mediterranean country geared up to celebrate its national day, with parades involving troops and military equipment through the centre of Madrid later on Sunday.
There were no reports of injuries as a result of the blasts, caused by two 1kg bombs placed under the lorries, a unidentified spokeswoman for the Ertzaintza Basque autonomous police force said.
Irun is located 480km (300 miles) from Madrid.
Asked if she thought ETA was behind the bombing she said, “Yes, they didn't leave a note, but we suspect it is them,” AFP reported.
ETA has launched attacks on this day for the past two years, firing grenades at a police barracks in the north of the country 12 months ago, and planting a car bomb in central Madrid in 2001.
The car was illegally parked and exploded after being towed to a garage near where Spanish Prime Minister Jose Maria Aznar and King Juan Carlos watched parades. The bomb injured several people.
ETA has killed more than 840 people since 1968 in a violent campaign for an independent state in the Basque areas of northern Spain and southwest France.
It is listed as a "terrorist" organisation by the European Union and the United States.