In two places along the northern portion of Ballynakilla townland, the present road quietly diverges from its predecessor, leaving short but legible stretches of an older surface exposed on the hillside and lower ground. These remnants belong to what is known as the Bealach Mór, a name meaning "great road" or "great way" in Irish, and their origin is a military one.

The Bealach Mór is believed to have been constructed in 1602 by the army of Sir George Carew, Lord President of Munster, as a means of moving troops efficiently across the island. The year is significant: 1602 was the final, grinding phase of the Nine Years' War, and Bere Island was strategically important, sitting at the mouth of Bantry Bay. The road runs broadly from Rerrin, near the eastern end of the island, towards its western end. Two surviving sections have been recorded in Ballynakilla. The first is a stretch of roughly 800 metres running northwest to southeast across the north-facing slopes of Knockanullig, between two and three metres wide. A second, shorter section on lower ground to the southeast measures around 70 metres in length and is slightly wider, at three to four metres. What makes these stretches particularly striking is a detail noted by researcher O'Sullivan: where the gradient steepens, the road incorporates large step-like features, often cut directly into the bedrock. This kind of rock-cut terracing is a practical engineering solution for maintaining footing on a slope, but it is also, four centuries on, an unusually direct trace of military labour.

Visitors to Bere Island who follow the road west from Rerrin are, without necessarily knowing it, tracing a route laid down under the pressures of a sixteenth-century war. The diverted sections, where the old road and the modern one part company on the slopes of Knockanullig, offer the clearest sense of what remains.