Pre-schools take children from age three, or your child can stay in nursey.
Some (most) nurseries also provide an educational element.
Every child is entitled to fifteen hours a week free education from the age of three, but it is not compulsory and so they do not have to attend at all, or you can pay for them to attend for additional hours. Also the 'free' element has a fixed value, so if you choose a private pre-school it is possible that you will still have to pay the difference between the published fees and the fixed value for the first fifteen hours.
Children normally start school in the September following their fourth birthday, regardless of when in the year they are born, but some schools allow chidren with later birthdays (April to August) to delay until the January after their fourth birthday.
Children either start in a primary school which has children aged between four and eleven, or an infant school which has children from four to seven who then transfer to a middle (junior) school at age seven until age eleven.
The different systems operate in different areas of the country and, in some towns, both systems may be in operation depending on which school you choose!
Secondary schools in the state system are for children aged 11 to 18, although in the independent sector the system is different again, as follows:
Pre-Prep, aged 2 to 4, Prep aged 4 to 13, Senior School aged 13 to 18.
Hope this helps, good luck.