Purpose: To systematically develop a farm safety intervention for farmer peer learning groups, to support safe working practice adoption. Design/Methodology/Approach: An iterative, multi-actor process was used, guided by the Intervention Mapping framework and the Socio-Ecological Model. The target peer learning groups were Irish dairy farmer discussion groups (DDGs). Literature review, semi-structured interviews, surveys, and participant observation informed an initial intervention design, which provided a starting point for collaborative intervention development. The final design comprised two intervention approaches, A and B, differing in frequency and discussion length. In a seven-month cluster-randomised, controlled pilot study, 76 DDGs implemented the design. Findings: The designs were standardised but adaptable to individual group interests and schedules. Pre-pilot feedback from farm advisors (intervention deliverers), and farmer and advisor recruitment, indicated good intervention acceptability initially. Challenges were experienced during implementation, but 52% of DDGs participating in the infrequent, long discussions approach ('A') and 72% of those in the frequent, short discussions approach ('B') completed at least some of the protocol. Practical implications: The multi-actor approach resulted in two flexible, acceptable designs, responsive to group culture and dynamics. Frequent, short discussions were acceptable to a greater number of groups than infrequent, long discussions. Theoretical implications: The Socio-Ecological Model, combined with multi-actor Intervention Mapping, provided a robust framework for the development of acceptable, practical, evidence-based farm safety and health promotion initiatives. Originality/Value: The systematic public health research-driven approach evolved iteratively to a DDG practice-driven approach, through integration of Intervention Mapping with collaborative multi-actor design.