Don't know anything about Irish schools but if the head teacher is just sitting and nodding but not really interested, listening and doing anything then is it worth taking a list of things in with you?
Like get her to explain why she didn't do anything last year or sort anything out, what did she think went wrong, in hind sight what does she think could have been done better?
And then on to this time, what is she planning on doing to punish the bully and to stop him from doing it again, what she is going to do to protect your son,
If you aren't able to take anyone with you, then take something in to record the session so you have got proof of what was said. Also write notes as you go - don't be embarrased to take the time to do this. At the end of each relevant chunk of converation, summarise back to her the main points that you discussed and -importantly - the actions that she is going to take to sort things out.
If there is any disagreements make sure each point of view gets documented and reasons why.
If you do it on sheets of A4 paper as you go, at the end of the session you can both sign it and then she can do a copy to keep on file - present it as saving her from a job of having to do notes too maybe. And then you can include the notes as an appendix with any letters sent and keep an on-going action list to use as the basis of future meetings.
I would also ask her - and maybe have a separate letter to back it up so it is definitely a request in writing too - that if any (serious) bullying incident occurs whilst you are not there, that they call you (don't know if you are able to drop everything and go up to the school) but that they call the police at the time of the attack. Might re-inforce how worried you are and will give you more ammunition if something happens (fingers crossed it doesn't) and they don't do anything about it...
Also it would show your son how serious this is, how seriously you are taking it and hopefully give him more confidence to be at school, if he knows all the teachers will be looking out for him.