This place has been visited by me and my girlfriend in 1999. The small cafe is situated in what you might call an alley rather than a street, and has a little terras of aproximately two to three tables. You can find the bar close to the river Douro in the area called Ribeira, which is the centre of the touristical city. With your back to the city, you take the last street on your right from the main square along the riverbank (Praca da Ribeira). This street has a shallow slope, and after several tens of meters you can find the Ribeirinha.
We visited the cafe twice on the same night. Earlier in the evening we sat outside. We noticed then, that several groups of young people would enter the place, drank a couple of shots of a particular spirit, and would leave again shortly after entering. With curiousity raised, we asked the owners what was going on. They explained the following ritual. The particular spirit served, named Matosinho, was a homemade mix of different drinks, including aquardente (the rest of the drinks they kept secret). The Matosinho makes the Ribeirinha a notorious place in Porto. Of course, we tested the spirit! And you know what? After a couple of shots, you'll reach an agreeable drunken state, and one of the side effects is an unexpected improvement of the portugese language. We experienced this the second time we enter the place that night on 01.30, and the Ribeirinha was packed. We stayed at the bar till about 04.00, and by that time the bar was still open.
The crowd in the Ribeizinha is a mix of strange people on the lose. The bar is owned by an old couple, that charmingly keep everybody in a constant drinking mode. The service on the terras is done by their son, or should we say a clone wannabe of Freddy Mercury. Later at night this Freddy type played the bouncer. Overall the Ribeizinha is a real sleazy place, waiting to be checked when you're around!