BackgroundDespite methodological improvements in clinical trial design and conduct more generally, methodological limitations persist in trials concerning mental health care. A qualitative Study Within A Trial (Qual-SWAT), embedded in the KARMA-Dep-2 host trial, will be undertaken to explore and gain an understanding of two methodological questions in randomised trials specific to mental health care: (1) what are the key barriers and enablers of participation in randomised trials in mental health; and (2) how can randomised trials become part of routine mental health care. These issues will be examined from patient-participant and clinician- / researcher-participant perspectives, in alignment with PRioRiTy research themes.MethodsA descriptive qualitative study design will be used. Data will be collected via one-to-one semi-structured interviews, conducted via Microsoft Teams. The interview data will be analysed using Braun and Clarke’s Thematic Analysis approach. One-to-one interviews will be conducted with three participant groups (N= 30): 1) host trial patient-participants (n= 10); 2) potentially eligible host trial patient-participants who refused enrolment in the host trial (n= 10); and 3) clinician- / researcher-participants who are associated with work on the host trial (n= 10).Ethics and disseminationEthical approval has been granted by St. Patrick’s Mental Health Services Research Ethics Committee, Ireland (Ref: Protocol 09/20). When the study is completed, a report will be prepared and submitted to the Health Research Board (HRB). Findings will be shared with the host trial team and study participants, and submitted for publication.Host trial registrationClinicalTrials.gov (NCT04939649); EudraCT (2019-003109-92). Official title: Ketamine as an Adjunctive Therapy for Major Depression - A Randomised Controlled Trial: [KARMA-Dep (2)].