Each module run every two years, so if you miss a module in your second year, you may have to wait two years in order to take it. If there is a module running the following year that you haven't taken, you may be able to that that instead, but this is not an automatic option. Please note that you cannot proceed to the MSc with only 5 modules, no matter how well you have done in them.
If you miss a module due to certified illness or suchlike, you will normally be able to take it (or a replacement) again without incurring an extra fee. Note that UCD does not usually recognise work obligations as being extenuating circumstances, so if your company requests you to skip an ASE module, please be aware that this is likely to cost you in both fees and time.