If you are coming from Lisbon, take the A5 highway to Cascais. At the end of the highway, take the exit to Birre and continue straight until you reach a roundabout, then turn right. In the next roundabout continue straight, direction Areia. Keep going straight until you reach the roundabout of Areia and take the second exit. You'll find a second roundabout, you go straight. You keep going until the end of the street and turn left. Continue straight in this sea-road until you see the entrance to Oitavos Dunes (5km from the intersection), marked with white flags, on your left.
Coming from Cascais, take the 'Avenida 25 de Abril' from the city centre. At the gas station follow the road to the left until you get to a roundabout. At the roundabout turn right towards the Sintra-Guincho direction and continue straight in this sea-side road until you see the entrance to Oitavos Dunes (ca. 3 Km from the roundabout), marked with white flags, on your right.
R. de Oitavos, 2750-374 Cascais - Portugal.