[2][5] The Lyreen then flows through Derrinstown, and under the M4 between Treadstown and Crinstown, meeting another tributary just south of the railway line as it reaches the edge of Maynooth.

The Lyreen then passes Maynooth Castle, built at its confluence with the Joan Slade (or Owenslade) River.

Running north-east, behind the house's of Maynooth's Main Street, the Lyreen receives the Meadowbrook Stream (also known as the Rowanstown River and Roosk Stream), which comes from townlands south of the canal, and flows in culvert by Straffan Road, visible on the approach to the town square.

Receiving the Maws or Crewhill Stream, it passes three artificial fishing ponds, then flows into the Rye River.

[5][2] The Lyreen has a modest fish population, with seven species detected in 2011, primarily minnow, brown trout and stone loach.