Sorry, the output won't be 230V.
An emitter follower looses 0.6V without a load, about 1V with a load and the 555's output stage looses between up to 2.5V so the voltage delivered to the primary of the transformer will be a couple of V lower than half the power supply voltage.
The transformer will also have more turns on the lower voltage winding than you'd expect, to make up for the losses. For example measure the secondary voltage from a mains transformer with no load connected and it will be up to 25% higher than the voltage rating, so a 6V transformer might measure 7.5V which means that you'll need 7.5V connected to get the rated primary voltage.
If you use a 12V power supply, you'll need to use a 3V transformer to get anywhere near 240V on the output.