You'll need to file an annual tax return - Form 11 most likely and this is for your income tax. You'll also need to consider preliminary tax. You'll need to consider vat if you are selling services/goods of more than €37.5/€75K per year (or expect to within the next 12 months). That is assuming that the sales are liable to vat in the first instance. General advice is get an accountant to do the first year or two and then copy it going forward.
So then from this year you’ll need to pay preliminary tax for the current year and balance out the previous year(though hopefully your preliminary has been a good guess and there isn’t much tidying up for the previous year) for your self employed earnings - no issue being a paye employee at the same time - have done this myself for many years - I found accountant well worth it when you find a good one.