The long, narrow plots running north and south from the street, known as burgage plots, are the parcels of land allocated to tenants in a planned plantation town, and in Kilbeggan their boundaries have survived largely intact since the seventeenth century. The continuous rear wall where those plots terminate may even have served as the town's defensive perimeter, though no document confirms it. The buildings themselves show nothing older than 1700, and yet the skeleton of a much earlier urban ambition is legible in the streetscape if you know what to look for.

The place is older than the plantation town by a considerable stretch. A monastery was founded here on the River Brosna by St Beagán, probably in the sixth or seventh century, giving the settlement its name. Around 1150 that early foundation was superseded by a Cistercian house dedicated to the Virgin Mary, stocked with monks from Mellifont in County Louth. By 1682, when the writer Henry Piers visited, even that abbey had vanished so completely that he noted there was "not so much as the rubbish to be seen." The house built on the dissolved abbey's lands by Sir Oliver Lambert had fared little better, burnt during the wars of the mid-seventeenth century and still unrepaired when Piers passed through. Lambert had been granted the abbey site, its gardens, a water mill on the Brosna, two eel weirs, and various lands in 1606, and in the same year received a licence to hold a Saturday market and an annual fair on the feast of St Michael. The town received its charter of incorporation from James I on the 27th of February 1613, and by 1618 an inquisition into Lambert's estate recorded sixty dwellings and sixty gardens within it. The Down Survey maps of 1656 to 1659, compiled during the Cromwellian land settlement, captured the town at a telling moment: a loose cluster of houses strung along the road between Tyrrellspass and Horseleap, the bridge over the Brosna already in place, the abbey ruins on their low hillock to the east of the river, and a tower house standing between abbey and church that was likely Lambert's seat. The same survey's written terrier, surveying the whole of Moycashel Barony, found little to celebrate but conceded that Kilbeggan was "the best" of what was there.

The rectangular widening of Main Street that forms the Market Square is a direct inheritance from those early market licences, and the burgage plot pattern makes Kilbeggan an unusually clear example of early seventeenth-century Irish urban planning. Older layers remain buried. A geophysical survey carried out in 2004 in the field west of the abbey site may have detected the boundary ditch of the original early Christian monastery, and the location of St Beagán's own church, thought to be somewhere near the old graveyard to the south-west of town, has never been confirmed by excavation.