From the airport, you can take the Express airport bus number X96 to Piraeus This is its final destination so don't get off en-route!
The Metro service is also very efficient although you do have to change to a different train half way through your journey.
Alternatively you can take a taxi from the rank parked outside the airport terminal.
There are also car/limo drivers who will collect you from the terminal as you come through customs and drive you to the port if you want a really personal service.
Michael Nakis 24 hours Airport Transfer can also help out, even if you are looking for local transportation around Athens.
Whichever option you choose to get you to Piraeus, 1.5 hours should be enough time regardless of the time of day (except of course if there are unexpected events).