Mullingar tells you about a miracle. The Irish name, Muileann Cearr, means the wry or left-hand mill, and according to a twelfth-century life of St Colmán mac Luacháin, the mill turned left-handwise because the saint caused it to do so. That is why the town is called what it is. The medieval documents add a further layer: many of them prefix the name with Ad- or Ath-, the Irish word for a ford, pointing to the crossing point on the River Brosna that was, practically speaking, the reason anyone settled here at all. The town's name is, in other words, two stories compressed into one word, a topographical fact and a hagiographical claim running side by side.

The settlement that grew up around that ford was Anglo-Norman in its formal origins. William le Petit, granted the manor of Magheradernon by Hugh de Lacy, is generally taken to have been the founding lord, and his motte, the raised earthen mound of a motte and bailey castle, a type of fortification introduced to Ireland by the Normans, was probably built before 1192. A parish church of All Saints followed shortly after, and William le Petit was granted a four-day fair in 1207. By the second quarter of the thirteenth century the town had acquired an Augustinian priory, founded around 1227, and a Dominican priory around 1237. A Frankhouse belonging to the Knights Hospitaller also stood within the town, and by 1297 there was a provost and a corporation. The medieval streetscape, essentially the corridor now known as Oliver Plunkett Street and Pearse Street, was the old highway from Dublin to Athlone, and the present-day Square may occupy the site of the medieval Cross mentioned in a 1612 land grant to Thomas Pettyt. By 1667, a confirmation of a land grant to Sir Arthur Forbes listed eight castles, three mills, two dissolved monasteries, and the earthen mound of the old Norman castle still visible in the town.

The most vivid single account of Mullingar in this period comes from Sir Henry Piers, writing in 1682. He noted the mill that gave the town its name, describing an overshot mill with a wheel so small that it and its buckets together measured less than eight feet in diameter, though it had by his time been converted to a breast mill. He observed that virtually every house in the town was an ale house, that it sat on the main road from Dublin to Connaught, and that its ancient corporation had recently been dissolved after the town and its commons were granted in fee to the Lord of Granard. The fifteenth and sixteenth centuries had been considerably rougher: the town was burned by the Uí Maine in 1484, threatened by Aodh Rua Ó Domhnaill in 1475, plundered and burned by the Clanricard Burkes in 1572, struck by plague in 1575, and burned again by Aodh Mac Uidir and Cormac Ó Neill in 1587. The seventeenth century brought stability of a formal kind, the town designated as the assize town of the county, it was said, for its better peopling.