* Tesco drops supplier of burgers containing horsemeat * Says to use DNA testing to avoid repeat of scandal * Move a blow to Ireland's $2.6 bln beef industry By James Davey and Stephen Mangan LONDON/DUBLIN, Jan 30 (Reuters) - Tesco fired a supplier controlled by Ireland's most powerful beef baron on Wednesday for selling it burgers containing horsemeat and promised to use DNA testing equipment to check the content of products on its shelves.