yeah they give you some more time if you don't have it, but you won't be marked as a registered student / can't receive exam results. Won't really affect your start, but you'd want to get it sorted asap after first registration day.
Also interms of whats it like? well 4 years there

and I loved it.
Downsides:
20min walk to blanch center....(there is an awesome shuttle bus that started this year every hour or quicker) but your still far out from the well serviced shopping center so it is a downside.
Pretty much the same point as above, depending on where your coming from it's poorly serviced by transport, this has been seriously helped with the shuttle bus but it's still extra hassle you have to deal with day in day out if your getting public transport
Parking, you have to get a permit for last year I think 50e or 80e for the year, and it doesn't guarantee you a parking spot... and the clamper's are very quick at spotting someone incorrectly parked.
In first year, they gave my year atleast, a really awful timetable on one day we had a 4 hour break then classes till 6. Half the class went home, and just missed that class, so skipping classes is an easy trap to fall into made easier by poor timetables. The gap I endured was entirely based on what group I got randomly placed in, so you can be lucky or very unlucky for the first semester.
Good stuff:
There's a good feeling about the place, you'll see almost the same people around during your entire time there.
The further you progress in years the better your timetable gets... and oh yes it gets much kinder ;D
Societies, ...remember those big breaks in timetables... yeah.. join a society, helps pass time when you need to do that and they also give you new friend circles outside of your class + you meet people who are interested in the same stuff.. so it's win win ;D
Lecturers... most lecturing staff will go out of their way to help you with lab times being difficult for you even just a simple question about some random topic in the class.
Advice:
Ask all the questions, stick the hand up and get help as soon as you need it in a lab, don't go home without saying anything, lecturers can't help you if you don't look for help.
Get all your work done as it comes to you, if you build up a good system of doing that you'll make it the entire way through 1st year without once doing work on a weekend or when you get home. In later years the work does pile up, so weekends will suffer

but not until your in 3rd or 4th year.
Attend everything....and I mean everything.. do not miss labs.. work builds up in stages.. miss one lab then attend the next one and your suddenly behind everyone, lecturers will help you but you'll have your work cut out if you keep up missing labs.
Plenty of mature students in my class.. so age difference shouldn't worry you if your a mature student.
Oh just noticed your into GAA, if you do go to itb talk to Ronan, he's a red haired chap whose a cool guy behind everything sporting at itb (gym/teams/socs) he should be your first port of call to get involved in anything sporty in the college. Pretty sure there was some emails about a new pitch opening a few weeks back.
Other then that... enjoy