The street plan was already established by the close of that century, and many of the long, narrow plots running back from the main street to the town wall, burgage plots as they were known, a form of medieval urban land tenure common across Anglo-Norman settlements, remain a structuring element of the town's layout to this day. Several of the buildings along Sarsfield Street are almost certainly seventeenth-century structures, though you would not easily know it: most have been covered in cement or plaster, concealing whatever architectural detail might once have identified them. Thomas Mulvaney's painting, now held in the National Gallery of Ireland, shows what some of these houses looked like before they were plastered over.

The town takes its name from a monastery founded by St. Mocheallog in the early seventh century, though by the tenth or eleventh century that monastic community appears to have migrated downhill from Kilmallock Hill, about a mile to the north-west, onto the low-lying ground the town now occupies. Curiously, when the Anglo-Normans arrived and the settlement grew into a proper medieval town, the principal church was dedicated not to its founding saint but to the apostles Peter and Paul. The base of a round tower survives within the fabric of that church, a reminder of the earlier ecclesiastical phase. The bishops of Limerick, who appear to have held Kilmallock at the time of the Norman arrival, were most likely the ones who laid out the medieval town, a pattern repeated elsewhere in Ireland, Cashel by its archbishops, Swords by the archbishops of Dublin. A collection of deeds preserved in the Black Book of Limerick, dating to around the late thirteenth century, records stone houses, a market cross, and the early defences of the settlement. Among those named is one Radulph of Kilkenny, owner of a stone house, though it is made clear that timber buildings were still the norm. The town later passed substantially into the hands of the earls of Desmond, and its strategic importance as an outpost of English government rule during the Munster Plantation made it a target: James FitzMaurice plundered it over three days in 1571.

The most visible survival of the medieval town is its wall circuit. The western wall runs for around 525 metres, and the Blossom Gate to the south remains the most complete feature: a tall structure with a rounded archway broad enough for single-file traffic, a narrow slit window above, and one surviving flanking tower, rounded on the outside. It has been compared to the Clock Gate in Youghal before that structure was later altered. The river Loobagh provided a natural element of the defensive circuit along the eastern side, supplemented by a lake to the west, so the fosse, or defensive ditch, was partly geography rather than construction. The walls themselves vary between one and two metres in height in their surviving sections, often cemented along the top, with lower remnants absorbed into later boundary walls elsewhere. Orr Street, once called Church Lane, still follows the old line from the main street to the churchyard.