If you want to travel between Piran and Portoroz or Lucija then this is very wasy and cheap.
There is a little white bus running 3-4 times every hour that costs a euro if you buy the ticket onboard and about half that price if you buy the ticket in advance at the little newspaper stands.
Don´t confuse that bus with the free shuttle bus which has a red and blue stripe that only travels to the edge of town.
The bus to Portoroz and Lucija is plain white and leaves from Tartini square just like the free shuttle bus.
There are buses that go to Portoroz from Piran from the main Piazza. Small shuttles that only cost 1 euro, so that seems pretty cheap. Same the other way around. I think that the last bus from Portoroz to Piran takes out at 23:49, but always check before hand!!
For those who want to go to Trieste from Piran, buses leave at 6:45, 11:25, 13:25, 15:25, 19:55
However, go check it out at the main bus station or at the Tourist Information Office.
It takes about 1:30 hours.
Local buses operated by I & I Koper connect Piran with other towns along Slovenia's Istrian coast.
The buses run frequently throughout the day on the following route:
Koper - Izola - Portoroz - Piran (and vice versa).
The journey from Koper to Piran takes 30-40 minutes.
As at April 2005, the cost of a one-way ticket between Piran and Koper was 670 SIT and the cost for a one-way ticket between Piran and Izola was 490 SIT.
Further afield, buses head north from Piran (via the other Slovenian coastal towns mentioned above) to Trieste in Italy. There are also a few buses each day which leave Piran and head south to various towns on Croatia's Istrian peninsular.
I took the frequent bus service from Koper, which runs along the coast road via Izola. They run every 20 minutes or so, and takes under an hour, so it makes an ideal destination for a day trip (or vice versa).