The geological time scale is shown in a chronological chart; it relates stratigraphy (layers of rock) to time and is used by geologists, palaeontologists and many other Earth scientists to time certain historical events on Earth.
As we have already seen in other pages in this section, the Earth is approximately 4.6 billion years old. This age is estimated by radiometric dating. Earth's past has been split into different sections based on events that happened during this time, for example, the boundary between the Cretaceous period and the Palaeogene period is separated by the Cretaceous - Palaeogene (formerly known at the Tertiary) extinction event, which marked the point where the dinosaurs and of many marine species went extinct.
The scale is split into different units; the biggest unit is the Supereon which is made up of Eons. An Eon is a period of time greater than half a billion years. Eons are split into smaller units called Eras which last several hundreds of millions of years. Eras are split into smaller again units known as Periods which are again split into smaller units called Epochs which are finally split into separate Ages.
Ireland's geological history spans from the Proterozoic to the Quaternary.
Geological time scale, source: http://www.rocksinmyheadtoo.com/TimeLine.htm