There is a pretty decent subway line that connects Pireaus to the downtown Athens area. If you go to this link http://www.athensguide.com/metromap.html
you can note the green line is in full operation (been running for years). Depending on the actual docking position of your ship the walk to the Pireaus station will be somewhere between 1/2 and 3/4 of a mile (its an easy walk through the pretty attractive port/city area. One warning about this line is that there has been a lot of problems with gypsy pickpockets working this line...so take special care of your valuables (a good idea anywhere in Europe).