Final Report of IRISCAST (IRIS-Carbon-Audit-SnapshoT), a sandpit project as part of the UKRI Net Zero Digital Research Infrastructure Scoping Project. The aim of this project was to perform an audit of carbon costs across a multi-site heterogeneous infrastructure (called IRIS) by collecting and analysing snapshots of actual usage. The project identifies where the carbon cost sits across the infrastructure and explores what the drivers are that influence carbon cost. The evidence collected will be used to allow for consistent reporting, evaluation and decision-making. This final report summarises the key findings and recommendations moving forward from IRISCAST in working towards a roadmap for carbon neutrality in the UK's digital research infrastructures.