Start with published mapping from Geological Survey Ireland, which shows subsoils, bedrock and depth to rock at a regional scale, then confirm it on the ground with trial holes or boreholes. Published mapping indicates what is likely; only excavation and testing confirm what is actually there.
Geological Survey Ireland publishes national datasets covering bedrock geology, subsoils, depth to bedrock, aquifers and geohazards, and these can be viewed online for any location. They are a good first indication of what to expect, but they are mapped at regional scale and cannot capture local variation such as a filled hollow or an old quarry.
Site-specific confirmation comes from trial holes dug with a machine, or boreholes where deeper information is needed. These reveal the actual layers, the groundwater level and, with testing, the strength of the soil for foundation design.