According to the United States Census Bureau, the town has a total area of , of which are land and are water, comprising 3.78% of the town. The northern slopes of Mount Monadnock occupy the southern part of town. The highest point in Dublin is along Monadnock's northeast ridge, where the elevation reaches above sea level at the town line. Roughly the western half of Dublin lies within the Connecticut River watershed, drained by tributaries of the Ashuelot River, with the eastern half located in the Merrimack River watershed, drained by tributaries of the Contoocook River.