Carbon ion irradiation of carbon nanotube (CNT) bundles to enhance mechanical performance is investigated using classical molecular dynamics. Strategies to achieve inter-tube cross-linking for improved shear response without a drastic reduction in tensile strength due to induced defects are considered. Deposition energies of 50-300 eV/ion, fluences of 4 x 10(13) to 2 x 10(14) cm(-2), and dosages of 2-60 MGy on 7-tube bundles are studied. Within 100-200 eV/ion, the level of cross-linking is directly proportional to dosage and therefore controllable. Lower energy irradiation produces smaller-sized defects so similar to 100 eV/ion is the preferred energy. More than 10 different types of cross-link and a variety of defects are created. The defect level becomes excessive if either the energy or the fluence is set too high. Extension to larger bundles however is significantly more challenging. In 19-tube bundles, similar to 500 eV/ion is required to form cross-links with the centre CNT, and at this energy careful control of fluence is required to avoid excessive damage. Thus ion irradiation for improving mechanical properties is best suited to small bundles. However, a scenario whereby small bundles are irradiated prior to twisting into ropes is suggested as a possible future method for producing macro-scale cross-linked CNT fibres. (C) 2012 Elsevier Ltd. All rights reserved.