121 Congress Street
all damned night!
Great restaurant, great food, good service, and Portsmouth is a late night town anyway. If you find yourself there, or can make the trip, the atmosphere won't disappoint, and the people and food won't either. Open all weekend (literally) and decent hours during the week, go at least once if you can!
Afterwards, there are plenty of people in bars nearby or just hanging out on the street, sometimes a dance troup will perfrom at midnight or a magician will try some tricks. Enjoy.