Any income you earn from the nixer will be taxed at 41%. When you register with revenue, you will be set up as a sole trader, but this will make no difference to the tax rate.
You could, in theory, set up a company, but because of the (presumably) relatively small amount of money involved and the onerous compliance issues with regards to companies, I would not advise it.
You could ask revenue to reduce your tax credits and cut off point allocated to your existing job, meaning you would pay more tax on your wages each week, but would have unused credits and cut off, which you could then use against your nixer income. This will not save you tax, it will only help with cash flow.
Your best bet is to get an accountant to look after your tax return and get specific advice from him/her.