Never part of the Roman Empire, Ireland was one of the last places in Europe to have real, formal towns, most of which were founded by Anglo-Normans in the late 12th century. In contrast, most of the towns and cities in England or France were founded by the legions in the first century AD, more than a thousand years earlier. While the Vikings constructed trading ports around the coasts from the ninth century onwards, giving rise to cities such as Dublin, Waterford, Cork and Limerick, inland the needs of the cattle-farming, self-subsistent natives were met by occasional fairs, held at focal points such as crossroads and fords over rivers. Several such places saw the foundation of 'informal' settlements and powerful chiefs' residences, later used as the basis for Norman towns, such as Dún Dealgan, now Castletown motte, Dundalk. The Vikings also produced Ireland's first coinage, while the Gaels reckoned value in cattle and slave girls.
The largest agglomerations of people in pre-invasion Gaelic Ireland would have been around the great monasteries, such as Clonmacnois, Kells and our own Monasterboice. Monasteries were however primarily religious, cultural and craft centres, not market towns in the (mainland) European sense. These centres and their religious art are the greatest creation of the native Irish civilisation and are now being proposed as World Heritage Sites.
From around the turn of the second Millennium, a wave of settlers spread out of north-west Europe, seeking land and building towns and cities in regions that had never been part of the Roman Empire. This new 'Holy Roman Empire' (as the core of the civilisation became known) was based on feudalism and the Roman Catholic religion. Land owners were granted land and rights by their lord, to whom they owed military service and obedience. The cultivation of corn, such as wheast, rye and barley, was core to this new settled, formal society, organised around the manors of knights and the vast estates of churches and monasteries. Towns sprang up to provide markets where goods could be bought and sold. Larger towns became the seats of bishops. Religion was essential to this culture, and some of the worst mass killers founded many churches and monasteries, endowed with grants of land, to pray for the salvation of their wicked souls. One of the regions invaded by this cereal-farming, settled, pious culture was Ireland.
The Irish church had been the subject of intense criticism by Rome for some time and saw major reorganisations during the early 12th century. One of the most significant changes was the arrival of Continental monastic orders, including Saint Bernard's Cistercians, the first pan-European corporate organisation since the end of the Empire, who arrived at Mellifont in 1142.
While the construction of the continental-style monastery at Mellifont predates the more intense effort to 'Europeanise' Ireland that began in 1169, most of the county's mediaeval remains owe their origin to the arrival of Normans from Britain.
All the county's major modern towns, like Ardee, Carlingford, Drogheda (originally two separate towns) and Dundalk are essentially Norman foundations, though some of the other Norman boroughs (11 in total) have since faded to nothing (for example Castle Roche and Castle Ring).