It'll depend when they start. Say school starts at 9. Junior and Senior Infants (the first two primary years) will finish at 1.30. Other primary year 1st Class through to 6th Class end at 2.30. (Primary is a year longer. It's like Scotland here.)
There may or may not be a breakfast club and after school. Generally there isn't.
lunch is a packed one. Eaten in classroom. A few schools have free sandwiches.
One thing that might different (depending on your current local primarys) is primary school children get homework each day.
Also there aren't SATs here. There's tests at the end of each year, but they aren't made big of. Drumcondras are more to monitor progress.
Secondary is started usually the achademic year they turn 13 in. Secondary school day will be something like 8.30 to 4 (Not exact that example). The junior cert year will fall a year before the school year they'd do GCSEs (hope I'm explaining that OK!!). Leaving Cert will be done the same time A levels might (if they do Transition Year - which may be optional).
I forget another difference - summer holiday are longer. Primary is July and August. In secondary it's June, July and August.
Also on the religion side of things. Whilst a massive amount of schools are Catholic on most schools the religion isn't particularly strong, especially if it's not attached to a parish. Though if you're are in a Catholic school communion year it'll be notable.
There's Irish language school. Lessons are taught through Irish in these. In all other schools Irish is a compulsory subject - in primary and secondary. There maybe an exemption if there's a learning difficulty. But even then it's difficult to get.