Just to clarify - you send must send in Form 11E or Form 11P by August 31st in order for Revenue to calculate your tax. If you send it in after this time, you will need to make the calculation and sign the self assessment panel (Panel O) yourself.
Having sent in your form by August 31st, Revenue send out your assessment for 2013 (sometime in September or October) stating that your liability for that year was for instance €5,000. You then pay the balance of tax owed for 2013 and preliminary tax for 2014 (based on their assessment amount) by October 31st.
Your safe harbour for preliminary tax is to match the previous year's liability. Otherwise you need to pay 90% of the current year liability, so guess work is involved.
If you underpay preliminary tax, Revenue can impose interest on the underpaid amount, from due date to date of payment. The current interest rate charged is c. 8% p.a.
If you are paying any sort of significant amount of tax you should be using the ROS Offline Application to make the calculation and compare this calculation to your assessment. If you can't do it yourself, it's probably money well spent to pay somebody qualified to do so.