If you are already paying tax in your PAYE job then any additional income after allowing for expenses & the taxi write off (capital allowances) will be taxed at your marginal (highest) rate of tax. So if, say you are already on 41% at work and if you earn an additional €1,000 from the taxi after all expenses/allowances, then that will be charged to tax at 41%. Ther may also be PRSI at 3% and Health Levy at 2% depending on total income.
If, however, your taxi business results in net expenditure, i.e. expenses exceeding income, then that loss is allowable against your PAYE income at your marginal rate . The reverse of the first scenario in other words.
You would need to register as self-employed with Revenue. Form TR1 is the item you file to register. The annual tax return is a Form 11. It would be helpful, to get a proper understanding of how the tax system works when you have a self-employment, to engage the services of an accountant. The cost is tax deductible and will save you tax in the long run.
Best of luck with it.