How to recognise them
Looks like moss.
Cooksonia is an extinct type of simple plant similar to a moss that lived in the late Silurian to early Devonian (415 million years ago. It is famous as the first land plant that had true "veins" (conductive tissue) to transport water and sugars around the plant. Cooksonia was tiny - only a few millimeteres high, like all land plants that lived at the time.
The oldest known examples of Cooksonia are from the Devilsbit Mountain in Co. Tipperary.