Erris is not a detour to add after Achill. It is a separate day, a full tank, and the decision to let the North Mayo coast be the destination. The road continues west beyond Bangor Erris toward Belmullet, then narrows again along the Mullet Peninsula, with Atlantic water appearing on both sides.
Elly Bay is the quiet release after the drive: a long strand on the sheltered side of the peninsula, low dunes behind it and winter light that can make the whole bay look silver. Continue only if the weather is holding. Out here, cloud and wind do not spoil the view so much as remove it, and there is no merit in collecting headlands through a windscreen.
Three nights in Mayo are not enough for this coast. Keep Westport, Achill and Cong, and leave Erris for another trip. With four or five nights, give the peninsula its own day and return before dark. Fuel in Westport, Castlebar or Belmullet when the opportunity appears. Long stretches of coast and bog in North Mayo ask you to be self-sufficient, especially on Sundays and outside summer.