The street runs east to west along the Avoca river, and on the Ordnance Survey map the narrow burgage plots, the long thin strips of land assigned to the town's burgesses under the medieval system of urban land tenure, are still legible, pressing back towards the river to the north and meeting what was probably a common defensive boundary to the south. The district across the water is called Ferrybank, a name that quietly preserves the memory of the crossing that preceded the bridge, when the only way between the two banks of the Avoca was by boat.

The town's origins are genuinely layered. The name itself is Scandinavian, one of only a small number of Irish place names that can claim that distinction with any confidence. The suffix "-lo" means meadow, though the prefix remains disputed; one reading gives "Arkell's meadow", another simply "river meadow". A ninth-century Viking burial found in the area confirms some form of Scandinavian presence, though the absence of documentary sources suggests it was never more than a hamlet or a stopping point on the Dublin to Bristol sea route. The town as such began with the Normans. Theobald FitzWalter received the manor from Prince John in 1185, and one of his first acts was to found a Cistercian abbey on what was described as the "island of Arklow". The Cistercians favoured remote, rural sites over urban ones, and the fact that the abbey was planted here at all suggests there was no significant existing settlement to displace. By 1205, Theobald had moved the monks to Abington in Co. Limerick, a relocation that likely signals the beginnings of an Anglo-Norman town taking shape in their wake. The castle was captured in 1331 during the Gaelic resurgence, and by 1571 the town was described as being in a state of considerable dilapidation. That year, an agreement between the earl of Ormond and the burgesses of Arklow set out new terms: rents were restructured, the burgesses were obliged to supply four labourers for four days each year towards repairing the castle and walls, and they were required, at their own expense, to construct gates, ditches, and pales against "rebels and evil-disposed people". They were also to maintain watch and ward by day and night and to send workers to mend the town's haven. Whether those defences were ever built is unknown; no physical remains survive, though the long southern boundary of the burgage plots hints that some form of earthen rampart and fosse may once have run there. The town held through the Nine Years War, sided with the Confederate cause in the 1640s, surrendered to Cromwell in September 1649, and finally passed out of Butler hands in 1700 when the family sold the manor to John Allen of Stillorgan.

Of the medieval fabric, only a fragment of the castle and the street pattern itself endure. The narrow lanes running between properties off Main Street are still there, though whether any of the buildings they serve contain fabric older than 1700 is impossible to say.