I used my Australian National licence at Chiang Mai without having an IDP. The transport authority had no problem with that and I was never asked if I had an IDP. At the same time I understand that many foreigners feel more comfortable if they have the IDP and that some Thai's (especially some police) mistakenly believe that an IDP is required to drive in Thailand.
There is no "international licence". There are only National Licences. Perhaps you had an International Driving Permit which is of no value by itself and is simply an English translation of a National licence.