Hibernia (Ireland) existed as a relatively isolated corner of Celtic culture during the Roman era. Hibernia, however, had a long standing pre-Celtic culture dating back to 6000 BC. A race of what some relate to the Pictish tribes of Scotland, called the Fir Bolg, dominated the island in Ireland's stone to late Iron Age periods. The first Celts arrived from mainland Europe, via Britain, likely beginning around 600 BC, and after a slow migratory trickle, began to flood the island between 350 and 150 BC. Bringing an advanced Iron Age culture, in comparison to Ireland's earlier inhabitants, the Celtic culture slowly surpassed the previous one and the Celts were dominant by the first century BC.
The Celts, or Gaels as they called themselves (hence the term Gaelic), were dominated religiously by Druidic customs which unfortunately limited the passage of the written word in early Ireland. Though later Christian monks would begin to record the ancient folk stories for posterity purposes, very little is known of Ireland's early history outside of anecdotal evidence. These stories, however, when studied along with archaeological evidence, do provide at least a comparative basis to understanding the basic culture of the time. Celtic culture in Ireland developed much like that of continental Europe and Britain. The people lived generally in small tribal families, sometimes loosely confederated in a larger cohesive unit in times of need, such as war. The Gaels, like their continental cousins, were typically predisposed to local autonomous government, as opposed to unified kingdoms or states. Small farming and self reliant villages were scattered throughout the island, and the Celtic people were highly regarded in the ancient world as masters of the metal smith arts. Much like their cousins in Gaul, the Irish Gaels were also a warrior based culture, where force of arms on a relatively small scale was a common measure of both individual and overall tribal status.
The Romans were vaguely aware of this edge of the world territory, likely due to Phoenician traders, but contact, at least knowingly wouldn't occur until the mid to late first century AD. To the Romans, Hibernia was an almost mystical place, thought to have been located somewhere in the Atlantic Ocean, between Britain and Spain. When the Emperor Claudius sanctioned the invasion of Britain in AD 43, which was followed by several years of native resistance, the Hibernians likely played a part in defending against Roman advancement. The Gaels, however, weren't simply just allies of British Celts because of cultural similarity; they also preyed on the western coasts of Britain, raiding native and Roman establishments alike.
While trade between the Roman and Gaels certainly occurred, as evidenced by the scattered existence of Roman artifacts on Ireland, little is known of real Roman contact and influence directly with the island. According to the Roman historian Tacitus, his father-in-law, Gnaeus Julius Agricola thought Ireland could be conquered with a single legion. Whether Agricola, governor of Britain between 78 and 84 AD, ever made an attempt on Ireland is unknown, but it certainly isn't mentioned by Tacitus, who left us a detailed biography. There is some evidence that the Romans did at least maintain a minor presence though. A fort at Drumanagh, on a promintory near Dublin, appears to be of Roman design and occupation. The presence of Roman artifacts at other locations, found alone and not in the presence of Celtic equivelants, would tend to suggest that the Romans were on the island at some point, if even for a short lived expedition. Consider that the short expeditions to Britain of Julius Caesar may never have been known to modern history, save for the fact that Caesar himself recorded the events. Regardless, the debate on Roman presence and influence in Ireland rages on to this day.
While that debate continues, it can be assumed that the bulk of Roman influence came from coastal trade. In the second century AD, the Greek geographer Ptolemy prepared the oldest known map of Hibernia, which included tribal names that also existed in Britain. British Celt migrations to Ireland, perhaps to avoid Roman rule, still would have subjected the Gaels to some Latin customs and traditions. 2 separate monarchies were established in the 2nd and early 3rd centuries BC, one in central Ireland and the second in the south. Influence from Britain clearly had an impact on the social and societal structure of the Irish tribes. Gaelic raids too certainly played a part in spreading at least a limited form of Romanization. In fact, one raid in particular turned out to have one of the most significant historical impacts on the culture of Ireland.
In the late 4th century AD, as Roman military influence in Britain and Western Europe was waning, the Gaels established a high king, by name of Niall of the Nine Hostages. Though his position must have been at least partially ceremonial, given many tribes aversion to a central authority, he was responsible for several raids on British soil. One such raid involved the capture of a young man by name of Patrick. At 16 years of age, around 405 AD, Patrick was brought to Ireland as a slave, where he stayed for six years, performing tasks of manual labor. In that time, the young man discovered Christianity, and after escaping to Britain, took up the cause of the church with zeal. After receiving a formal Catholic education and priesthood in Britain, he returned to Ireland as a Bishop in 432 AD, and steadily spread the conversion of Christianity to the population.
Patrick, later Saint Patrick the Patron Saint of Ireland, made a profound impact on the social and political environment of Hibernia. In spreading the highly organized and systematic Catholic Church, he spread a measure of Romanization that had never occurred there. With the coming of the Church, and the eventual fall of the Druids, literacy and the written language spread as well. Monasteries and schools were established, and even some members of the resistant Irish nobility were converted during Patrick's lifetime. While the rest of the Western Roman Empire crumbled, at the time of Patrick's death in 461 AD, Ireland was finally just becoming the ideal Romanized 'province'. By the time the Imperial government in Rome had fallen, and Western Europe was entering a 'dark age', Ireland was being enlightened by its new found faith and literacy and entered into its own 'Golden Age'.