The Shirebourn is a lesser river of the Shire. Its source is in the uplands of the Green Hill Country, from where it flows south and then southeast, where it matches the boundary with the Southfarthing. At Willowbottom, it meets the Thistle Brook, and then flows eastwards.
The Shirebourn flows into the Brandywine beneath Deephallow, and at its mouth forms the boggy region known as the Overbourn Marshes, opposite the southern end of the High Hay. Its outflow is known as the Mithe.
The Shirebourn's main course is not yet in-game, and will probably be included in a potential Southfarthing region whenever that gets released. The only accessible part of the river is its very end, which flows into the Brandywine opposite and a bit south of the Withywindle in the Old Forest.
The point at which where the Shirebourn flows into the Brandywine can be seen from the opposite side of that river, at the edge of the Old Forest.