The drive from the airport to San Isidro is fairly easy, and at that time of night should be no problem. If you made the drive before it should be very straightforward, just keep your eyes open at night to read the signs off Avenida Las Americas that point towards San Isidro, I believe the signs say: Charles de Gaulle - San Isidro and Ave. San Vicente De Paul....if I remember correctly these signs/intersections are all very near each other so just be sure you get on the right street.
From taking the new highway to Samana before I believe there is a connection from San Isidro that will eventually merge you on the new highway, however, I have not personally taken this road before and even so still think it would be faster to just get back on the highway [Las Americas] and go east again towards the airport and connect to the new highway to Samana. It will definately be faster than taking a backroad!
