Pre-historic structures like Newgrange or The Ceide Fields can be dated to thousands of years before Christ. Anthropologists can push the clock back further with evidence of even earlier societies.
Palaeontologists have found fossils dating back hundreds of millions of years to the dinosaur days and Valentia Island boasts some 400 million-year-old fossilised animal tracks. Geologists classify time periods in ages, epochs, eras and eons. Ages are rarely over 10 million years and eons range from about 500 million to 1,500 million years or 0.5 to 1.5 Giga years (Ga). We can use tree rings and silt deposits in lakes for younger samples but there are three methods of dating rocks of more than a million years old. Fossils laid down at different depths provide a "fossil age" with older species found at increasing depths in the geological column.
While fossil dating became the norm in the 19th century radiometric dating developed in the 20th century. These techniques use abundance and ratios of radioactive isotopes of a known half-life and of daughter elements arising from decay.
Periodic reversals in the earth's magnetic field, discovered in the 1960s, also offer a method of dating both igneous and sedimentary rocks, providing an additional tie between the other two methods.
Early traces of life called Stromatolites have been found in Shark Bay off the most western point of Australia. These date to 3.45 Ga.
The earth is a little over 4 Ga old and the oldest rocks we know date to between 3.8 and 3.96 Ga and are found in the centre of continents.
Anything older predates the planet earth so we have to look into space if we want to look further back in time. Various bits of "space dust" strike our atmosphere on a regular basis. If you go up on your roof and sweep the dirt out of your gutter you may be able to pick out small metallic pieces with a magnet. These drifted down from space.
The age of the solar system is reckoned at just over 4.5 Ga. Some of the rocks and stony iron meteorites, which land on earth (called meteors before they land), have been whizzing around the sun for at least 4.5 Ga. The elements in many meteorites match elements in the spectrum of light reflected from asteroids which suggests their possible source.
New missions to comets and asteroids already yield much information on the early solar system but the best physical evidence of material that predates the earth is contained in rocks which orbited far from the sun and were not significantly physically altered.
One very rare type called a Carbonaceous Chondrite contain grains of material which predate the sun.
It is impossible to tell how old these grains are but it is assumed they are about 5 Ga.
On the ground floor of the Natural History Museum on Merrion Square is a collection of space rocks including some moon rock. The collection includes one called Murcheson, a Carbonaceous Chondrite. This rock is certainly the oldest thing found anywhere in Ireland, dating back a remarkable 5,000 million years.