International Fusion Materials Irradiation Facility—Demo Oriented NEutron Source is an accelerator-based neutron irradiation facility for the study and qualification of materials used for the design, licensing, and reliable operation of fusion reactors such as DEMO. Neutron serves as a critical tool for irradiating and testing material samples, while also posing significant radiation safety considerations. This work provides a comprehensive overview of neutronics studies and nuclear analyses, beginning with the accelerator where neutron and gamma are generated from deuteron losses and deposition, progressing to the test systems where most d-Li neutrons are produced, and then to the systems and areas where the radiation is covered. Key aspects analysed include neutron-induced material damage and gas production, nuclear heating, radiation doses during both beam-on and beam-off conditions, activation of structural components and liquids (such as lithium and water), and the assessment and optimization of radiation shielding across the accelerator systems, target systems, lithium systems, and building and plant systems. Additionally, this article highlights recent advancements in neutronics simulation tools, evaluations of nuclear data, and experimental efforts, all of which provide essential foundations for nuclear analyses in DONES.