Ptolemy's map of Ireland: a modern decoding
Abstract Claudius Ptolemy (roughly AD 90–168) presents us the oldest surviving account of Ireland in the form of a set of coordinates showing different geographical features or a ‘virtual map’. While Ptolemy's map looks roughly like today's Ireland there are a number of discrepancies. Likewise, while some locations are obvious, others are disputed or obscure. After reviewing earlier attempts at reconciling Ptolemy's map with modern ones we adjust Ptolemy's coordinates to modern references with two equations. We find Ptolemy's map is consistent with some intimate Mediterranean knowledge of Ireland, its peoples, coastal features and principal places with their respective locations. We correct some of the modern locations attributed to Ptolemy, offer possible explanations for certain disputed or uncertain locations and offer some external validation to the prehistoric division of Ireland passed down from pre-Patrician sources.