Rome never invades. Rome does write Ireland down. About 150 AD the geographer Ptolemy of Alexandria maps 'Iouernia' with its rivers, its 'cities' and sixteen tribes, from the Robogdii on the north coast to the Iverni in the south. It is the first written snapshot of the Irish. Meanwhile raiders, traders and Roman coins are crossing the sea in both directions.
Irish 'Scotti' join Picts and Saxons in a coordinated assault on Roman Britain.
Quick answersWhen was Ptolemy's Ireland in Ireland?
Ptolemy's Ireland spans AD 100 to AD 399 in this atlas. Rome never invades.
Who ruled Ireland during Ptolemy's Ireland?
Power was divided: Eblani (4 counties); Voluntii (Uluti) (3 counties); Erdini (3 counties); Nagnatae (3 counties); Usdiae (3 counties), among others. County boundaries are modern approximations.
What is a lesser-known fact about Ptolemy's Ireland?
Ptolemy's map holds a genuine mystery: Nagnata, a 'notable city' in the west of Ireland — in an island that had no cities. Sligo Bay? A great fair-site? A misplaced monastery of rumour? Two thousand years on, no one has found it.
↗ links go to the relevant Wikipedia article for each event and ruler (sources & further reading).