You never get an offer for a course you placed below one you already qualify for. The CAO works down your list, finds the highest course you are eligible for, offers you that, and stops. Everything below it is irrelevant that round.
This is why the order of your list is the only tactical decision that matters, and why putting a course you want less above a course you want more can only hurt you.
The rounds
Round A and Round Zero come before the Leaving Certificate results and go to specific groups: mature applicants, some further education applicants, and others whose qualifications are already known. If you are waiting on Leaving Cert results, these are not for you.
Round One is the big one. It issues after the results come out in August, and it is where most school leavers find out where they are going.
Later rounds run through September. Places open up as people decline, accept elsewhere, or do not respond. A course that was out of reach in Round One can come back in Round Two at lower points, because the points are just wherever the last accepted applicant landed.
Exact dates and times are published for each cycle on cao.ie. The key dates page has the full calendar.
Offer morning
Log in and read the offer properly before you do anything. It names one course. That is the highest course on your list you qualified for.
Then check the acceptance deadline, which is on the offer itself. It is short. Days, not weeks. Miss it and the place goes to somebody else, and there is no appeal for having been away.
If you are happy, accept. Do it that day rather than leaving it, because deadlines have a way of arriving during a family holiday.
If it is not what you wanted
You have three real options and one bad one.
Accept it and stay in the running. Accepting a lower choice does not take you out of consideration for the courses above it in later rounds. This is the option most people should take, because it means you have something.
Wait for a later round. Only sensible if there is a genuine chance, which usually means you were close on points for something above it.
Look at another route. A PLC year, an apprenticeship, or mature entry later on are not consolation prizes. The routes page sets out what each one actually involves.
Declining because you are disappointed is the bad one. Sit with it for a day. The place will still be there tomorrow, the deadline permitting, and the feeling will have changed.
Deferring
Some courses allow a deferral, most commonly for a year. It is not automatic and it is not handled by the CAO: you apply to the institution directly, usually very soon after the offer, and they decide.
Two things to know. A deferral is for the course you were offered, not a general credit you can spend elsewhere. And you normally cannot apply to the CAO for the same course the following year while holding a deferral for it.
If you are thinking about deferring, ring the admissions office the day you get the offer. They will tell you their process in two minutes and it will be more accurate than anything you read online.