One of the things many visiting international students notice when they first study abroad in Ireland is that they show up at the scheduled time for a class, only to have to wait 10 minutes or so for it to begin.
Lectures are usually timetabled to last an hour, however it is generally understood that teaching begins at about 10 minutes after the hour.
The main reason is because students may have lectures in different buildings throughout the campus, and therefore may need the time to get from one to another.
Of course, some lecturers may begin on the hour, so it’s probably best to gauge each one individually over the first few weeks. However, if you do find yourself running late, there’s usually no problem getting in a few minutes after the hour, even if this would be not allowed at your home university.
***Update, August 2013 From the 2nd of September 2013 9am Lectures will begin on the hour and finishing at 10 minutes before the hour i.e. 9am-9.50am, 10am-10.50am etc.***