Give a list of every individual occurance you can think of that has happened; big AND small - because together the whole list builds up the big picture. Don't worry if it gets really long, that's kind of the point.
Do a paragraph for each incident. Try to give dates if you can, even rough ones likes "October 2009" or even "Autumn Term 2009". Outline what happened in the incident, who witnessed it and what the outcome was. Detail your son's response, including his emotion if appropriate, but keep your own emotion out of it at this point.
After you've catalogued them all, then write about the contacts you have had with the head teacher on this subject. List anything she promised to do, or don't be afraid to say "Mrs X was not able to give me any information on this occasion as to what the school would do to protect DS". Again, try and keep your emotion out of it at this stage. You don't need to say things like "I was furious when she suggested this....." - her lack of action is testament enough to how crap she is. At this point, don't muddy the waters by sounding overly emotional yourself. You want to sound in control and objective at this point.
After you've catalogued all your contacts with her, then move on to say that, because you had tried extensively to work with the school to no avail, you then tried to seek help from alternative sources. Then you detail every conversation you have had with the Guard, the anti-bullying people, etc. What they have suggested you do and what their advice was.
Then you move on to talk about the effect this is having on your son, your whole family, and you. This is the bit where you can say what a detrimental effect this is having on all your mental and emotional health. Bring in at this point the conversation you had with your GP and what he said. At this point you CAN express how angry, let down, etc you feel, and let them have an insight into how this is impacting on you all.
Then take some of the legal stuff people posted about about schools responsibility to safeguard children from bullying. Google the relevant passages on the internet and quote them in your letter.
Then end with a reasonable paragraph about how you hope that they can see that you really have exhausted every option open to you, and that you need them now to act on your behalf to put a stop to this. Here you want to aim to sound reasonable but firm and determined.
Copy the letter to your local paper, to your local MP (sorry don't know Irish equivalent), to your GP (get them to write in support of you too), to the LA and to the Guards. Write at the bottom of your letter, after your signature, each person you are copying it too, e.g.
Yours sincerely,
Casserole
cc: Mr Smith, MP
Dr Gilbert, GP
Mr Bloggs, Chief Education Officer, LA
etc
Seriously, I used to work reasonably high up in education for an LA and copying in those people does get shit hitting fans, but they need to see that you have copied in those people. Try and find a contact person for each establishment - ring your MP's office and get his/her name if you don't already know it, and ring your LA and ask for the name of the Director of Children, Schools and Families (or whatever your equivalent is)
Hope that helps.