There is a bus stop with a shelter at the end of the Promenade, on the side of the Primary School ( Scoill Phurt le Moirrey). Buses run to Castletown, the Airport and Ballsalla and Douglas via Port Erin or Shore Rd [most buses continue along the Prom in Douglas to the Electric Railway and Onchan].
Monday to Saturdays for most of the day there are two buses an hour to Douglas. One turns right and goes via Shore Rd, Gansey (this is the faster of the two). The other goes via Port Erin and Colby. Both routes join just before Castletown.  Sundays are similar to the Monday to Saturdays timetable but with slightly less buses in the early morning.
A summary of buses between Port St Mary and Douglas is below.

Also the number 28 runs to the Sound and Port Erin.  This runs on Saturdays and Sundays during the summer (there is no confirmation yet that this will run in 2024)

TT Bus Services

TT 2024 Bus Information

Southern 100 Races

When the Billown Circuit (Southern 100) Castletown to Ballabeg is in use, buses will only run between Douglas, the Airport and the Square in Castletown (passing the Steam Railway Station). There will be some buses operating between Douglas and Port St Mary via Foxdale (not serving the Airport, Castletown or Ballasalla).

The Steam Railway will operate as normal.

Blackford's Pre-TT Classic Road Races: 24th, 25th & 26th May 2024 - Bus alterations

Isle of Man Steam Packet Company Southern 100 Road Races: 8th, 9th, 10th & 11th  July 2024

All buses are run by Bus Vannin
The current island bus timetable is available here.

Check the Bus Vannin facebook page for last minute changes to schedule

Steam Railway

The Steam Railway Station is a 10 minute walk from Mallmore in the Port Erin direction.  Trains run most days between March and October.  Generally there 4 journeys each way and on occasions 6.  During TT race week a commuter service with restaurant car runs.
Steam Railway Timetable
More information here.

The Manx Electric Railway, Snaefell Mountain Railway and
Douglas Bay Horse Tramway

The Manx Electric Railway (MER) starts at Derby Castle at the north end of Douglas Promenade.  The MER travels through the lovely Manx countryside to Laxey and Ramsey.  At Laxey the Snaefell Mountain Railway (SMR) runs to the summit of Snaefell.
The Douglas Bay Horse Tramway runs between the Villa Marina and Derby Castle
Most of the Douglas buses from Port St Mary travel along Douglas Promenade to Derby Castle or the Villa Marina for the Horse Tram.  Alternatively take the Steam train to Douglas, walk along to the Promenade and catch a bus to Derby Castle or the Villa Marina.
Manx Electric Railway (MER)Snaefell Mountain Railway (SMR)
Douglas Bay Horse Tramway (DBHT)
Further Information