Yes, this would not be unusual. You'll need to pay tax under the self assessment system if you earn more than €5K from your business per year. For tax purposes your taxable profit is added to your salary and you are taxed accordingly. If you earn less than €5K you may be able to get away with filing a Form 12.
I'd suggest you talk to an accountant to help you with your first few years tax years - preliminary tax, commencement provisions, commencement to trade, basis periods, years of assessment, taxable profits, capital allowances etc are tricky enough. If you have enough time and interest you'll find all the relevant material on the Revenue website.
Watch your vat registration requirement depending on your turnover.