There is a lack of interventions for specific phobia in children and adolescents with moderate to severe intellectual disabilities. The objectives were to: (a) develop an intervention for specific phobia, together with an intervention fidelity checklist and logic model, and evaluate candidate outcome measures, together with parents/carers and clinicians; (b) describe treatment as usual; (c) model the intervention to determine the acceptability and feasibility for all stakeholders, judge the appropriateness of outcome measures, explore recruitment pathways, and examine the feasibility and acceptability of consent and associated processes; and (d) describe factors that facilitate or challenge the intervention. Phase 1a: using consensus methods, an Intervention Development Group was established who met to develop the intervention, review candidate outcome measures and contribute to the development of the intervention fidelity checklists and logic model. Phase 1b: a national online survey was conducted with parents and professionals to describe treatment as usual. Phase 2: a single-group non-randomised feasibility study was designed to model the intervention and to test intervention feasibility and acceptability, outcome measures and aspects of the research process. Phase 2: participants were recruited from National Health Service community child learning disabilities teams and special schools in England. Treatment was delivered in the child learning disabilities teams. Children aged 5-15 years with moderate to severe learning disability and specific phobia, and their parents/carers. The SPIRIT intervention comprised two half-day workshops and eight support sessions plus treatment as usual. The feasibility and acceptability of the intervention and research processes, recruitment, outcome measure completion rates and acceptability, and intervention adherence. Parents completed all of the outcome measures, with very low rates of missing data. The recruitment of sites and participants was impacted by the COVID-19 pandemic. The intervention was successfully developed and modelled with 15 participants with moderate to severe learning disabilities and their parents. The intervention was judged to be feasible and acceptable by parents/carers and therapists. Parents/carers and therapists suggested minor intervention revisions. Randomisation was not modelled within this feasibility study, although the majority of parents and therapists indicated that this would be acceptable. The SPIRIT intervention and associated study processes were judged to be feasible and acceptable. The intervention requires minor revisions. The SPIRIT intervention should be tested further within a clinical trial. Current Controlled Trials ISRCTN34766613. This award was funded by the National Institute for Health and Care Research (NIHR) Health Technology Assessment programme (NIHR award ref: NIHR130177) and is published in full in Health Technology Assessment; Vol. 28, No. 64. See the NIHR Funding and Awards website for further award information.