Getting to Mirissa
The cheapest and most obvious route is by public bus. About 100 rupees from Galle bus station. Buses are frequent (every fifteen minutes). The journey takes about an hour or so.
No need to book accommodation in advance as there's plenty of room in the low season. Downside of the low season is that the seas are too rough to go out whale-watching.
If you want cheap local food and avoid the slightly more pricy places in the beach, you'll find a great curry and kottu shop at the junction of Matara and Udupila Road.
Technically it's the monsoon, but practically, it rains only for a while each day, usually in the late afternoon. So sunsets are non-existent, but mornings are just about perfect.