I put away 50% of my earnings. That covers 41% tax, 5% PRSI/Health and new levies (at 1.66%) - that's too much to put by, but it gives me a comfort zone. I'd rather have put away too much than too little. Also, put away your VAT payments any time you get paid.
You can claim petrol for business but you have to keep records of mileage and calculate exactly what portion is used for business. You can claim phone bills, but again, only for business purposes. If, for e.g. you pay €25 a month for your phone rental and 50% of your calls are business-related, then you can claim €12.50 per month. Keep all receipts.