This is going to take some working out, but let's get started.
1. There are 3 wires in both your boat's 110v circuit and the dock's 220v circuit--a load wire, a neutral wire, and an earth wire.
2. In the boat circuit, the reverse polarity light goes between the neutral wire and the earth wire. Under normal conditions, the neutral wire should be within a few volts of the earth wire, and the indicator will not light. If the neutral and load wires have been switched, the light will see 110v, and will come on.
3. Now introduce a stepdown transformer into the circuit. Note that there are two types of stepdown transformers--one where the two sides are completely isolated from each other, and one with a common neutral wire, called an autotransformer. We don't know yet which type you have, but autotransformers are cheaper, and if you disonnect all the wires and find continuity between the input and output windings, you have an autotransformer, and an open circuit indicates an isolated transformer.
4. If you have the isolated type of transformer and you haven't grounded one side of the output, the voltages on both the 110v neutral and the load wires will float around, and both of them could show significant voltage differences from the earth wire, making the light come on no matter which way you have the wires (in the plug
5. If you haven't hooked up the earth wire from the dock
, the boat earth could be also floating around, making the reverse polarity light come on. Being careful, check the voltages between all three wires on the dock side and on the boat side. If you don't feel comfortable doing that, its time to call an electrician.
6. The dock circuit breaker could be tripping due to a ground fault, which is more complicated to deal with, especially if you have an inverter/charger on board. You should also hook up a 220v pilot light between the earth and the neutral input to the transformer, which will tell you if your dock power has reversed polarity.
That's a start, but it will take some doing to sort everything out. The average Irish electrician may have a hard time with this one.