Local controllable modification of the electronic structure of carbon nanomaterials is important for the development of carbon-based nanoelectronics. By combining density-functional theory simulations with Ar-ion-irradiation experiments and low-temperature scanning tunneling microscopy and spectroscopy (STM/STS) characterization of the irradiated samples, we study the changes in the electronic structure of single-walled carbon nanotubes due to the impacts of energetic ions. As nearly all irradiation-induced defects look as nondistinctive hillocklike features in the STM images, we compare the experimentally measured STS spectra to the computed local density of states of the most typical defects with an aim to identify the type of defects and assess their abundance and effects on the local electronic structure. We show that individual irradiation-induced defects can give rise to single and multiple peaks in the band gap of the semiconducting nanotubes and that a similar effect can be achieved when several defects are close to each other. We further study the stability of defects and their evolution during STM measurements. Our results not only shed light on the abundance of the irradiation-induced defects in carbon nanotubes and their signatures in STS spectra but also suggest a way the STM can be used for engineering the local electronic structure of defected carbon nanotubes.