IGH 2 - Precambrian to Devonian Palaeontology Theme
The sites selected for NHA designation in this theme cover a significant range of palaeontological interest and importance. No genuine Precambrian fossils are yet known from Ireland, and only a small handful of sites are known with significant Cambrian fossils. These include Bray Head with its distinctive Oldhamia trace fossil, and the Rocky Valley where microfossils have been used to correlate with other countries and define a precise age.

Many of the sites are Ordovician in age, when a major radiation of fossil groups occurred and the variety of environments to occupy was greater. Shallow seas around volcanoes provided suitable conditions for many different phyla of animals, often strongly biogeographically different from those of other plates at the time. These differences in faunas help us understand the major story of Ireland's two halves.

Silurian seas were also rich in fauna. Shallow seas had rich benthic faunas of corals, brachiopods, trilobites and molluscs, such as in South Mayo. Changing faunas here show a transgression or spreading of the sea over land, as deeper water communities replace shallower ones up through the succession. In deeper water areas such as Balbriggan, planktonic graptolites dominated the ocean. At one locality, Borrisnoe, in County Tipperary, graptolites help date the oldest known land plants in the world - a site of great significance.

The Devonian world was very different with a large supercontinent Gondwanaland, being quite arid and having fewer environments in which fossils could be preserved. Apart from Kiltorcan fossils, most localities have plant fossils, as the land surface had become well vegetated by this time. Many of these plants produced spores which can be invaluable for dating and correlating otherwise unfossiliferous sequences, through Munster and other areas of Ireland.