There are very precise rules about how this is calculated.
Basically you record all the costs of operating the car, being careful to keep the receipts for 6 years, and then deduct the appropriate % from your income as a cost. E.g. if the car cost €5k to run for the year and travelled 10,000 miles. Decide what % is relevant to the business, this will probably be an estimate, Revenue will probably accept 75% if you can demonstrate a high level of use. They will not accept 100%
The cost of buying the car gets tax relief as a capital allowance, the rules are detailed see Revenue site.
Clothes, as opposed to workwear e.g. safety boots helmets, are frowned on by Revenue. They will not accept anything which can be worn outside the work environment.