Most towns established under the Plantation of Connacht were at least nominally fortified, yet the Down Survey barony map of 1656 to 1658 depicts this small community on the east bank of the River Shannon as entirely open. By that point the town had already changed hands violently, captured by Roger Maguire on behalf of Owen O'Neill in 1648 and surrendered to Cromwellian forces four years later. A census-style count around 1659 recorded a population of just 47. By 1683, when a wooden bridge had finally been built across the river, only fourteen families were living there.

The town had been incorporated by royal charter in 1613 under the name Carrick Drumrusk, though a fort predating that charter had been constructed in 1611, with Maurice Griffith recorded as its constable from 1620. The fort's strategic importance faded quickly after the plantation town of Jamestown was founded further south along the Shannon in 1622, and a second fort had appeared on the Roscommon bank by 1627. The street pattern that gradually took shape tells its own story of slow, incremental growth. Bridge Street, curving toward the river crossing, formed the spine of the original layout, and the widening of that street just outside the castle site is thought to mark the position of the earliest market place. Main Street came later, probably toward the end of the seventeenth century, its alignment inferred from the position of St George's Church of Ireland. The market square at the northern end of Bridge Street was a later addition still. Archaeology has complicated the picture further: excavations up to 2000 failed to find physical traces of the town's earliest occupation, largely because nineteenth-century attempts to raise the ground level had buried everything under two to three metres of mixed organic material, stone, and red brick. More recently, monitoring of a water main uncovered the original quay-face and a redeposited oak beam with a probable felling date in the late seventeenth century, possibly a remnant of the 1683 bridge or its replacement, built in 1718.