Improvement of single wall carbon nanotube (CNT) bundle mechanical properties through carbon ion irradiation is investigated using molecular dynamics. Increased inter-tube shear and toughness properties through formation of inter-tube cross-links is balanced against decreased tensile strength from induced defects. Bundles irradiated with carbon ions with energy 50-300 eV/ion, and fluence between 4 x 10(13) cm(-2) and 2 x 10(14) cm(-2), are mechanically tested. We find that with careful control of irradiation parameters, shear and toughness parameters increase by an order of magnitude, while tensile properties reduce by only 30-40%; in real CNT fibres with discontinuous CNT filaments the reduction would be much less. The nano-scale interface response resembles that of micro-scale composites, in which interstitial C atoms play a key role. This makes C ion deposition an attractive option over irradiation by electrons or other types of ions, since the extra C atoms can provide the required interstitial atoms. Within a certain cross-link density range, the interface shear modulus, shear stress at bonding onset, and frictional sliding stress after debonding are all linearly related to cross-link density making controlled design of fibre shear properties feasible. A possible post-treatment with very low energy irradiation is proposed for healing holes and partially restoring tensile strength. (C) 2012 Elsevier Ltd. All rights reserved.