I was in exactly the same scenario just a few months ago. Its fairly straight forward(having liased with family accountant) - its simply classed as additional earnings. You can download the form from the tax/revenue website and must submit it within 90 days of starting your FI work. Then in April, submit a self tax return. Make sure you keep a log of everything including things like mileage to/from the airfield etc. The form only takes 2 mins to complete.