Start with the OPW's national flood information website, which shows both predictive flood maps and records of past flood events. Supplement it with the local authority's strategic flood risk assessment, local knowledge, and a physical inspection of the site and any nearby watercourses.
The OPW's flood information service brings together recorded historic flood events and predictive flood extent mapping produced under the national flood risk assessment programme. It is the primary public source and lets you view a location and see whether flooding has been recorded nearby.
Because national mapping cannot capture every local event, it is worth checking the local authority's strategic flood risk assessment in the development plan, asking neighbours and the local authority about past events, and inspecting the site for signs such as debris lines, culverts and drainage ditches.