You asked

How is ADHD diagnosed?

ADHD may be considered if your child is exhibiting a number of the symptoms outlined before the age of seven and for a period of more than six months.  Children with ADHD won't always be hyperactive but will still find it difficult to concentrate. If your child finds it difficult to concentrate or pay attention, but is not hyperactive, it may be considered as ADD (Attention Deficit Disorder) rather than ADHD. 

Early diagnosis and treatment has a massive psychological, social and educational impact.

If you first decide to visit a GP, your child will more than likely be referred to a child psychiatrist who, after an assessment will be able to diagnose the condition. It is often the child's teacher who will first speak to the parents about a possible problem. Providing the parent's give consent, the National Psychological Service (NEPS) will then be contacted. A psychologist will then be sent to the school to assess the child and determine a diagnosis. It is important to be aware that there are extensive waiting lists for this service and it is not available in secondary schools.

More questions

Your child will have an Individual education plan that outlines exactly what services your child will receive.
Any child who received special education resources or support in primary school will almost certainly be eligible for the same support in secondary schools.
If your child has been receiving extra help in primary school it is important to look for a secondary school that will suit his needs
There are significant differences between children who are slow learners and children who have a learning difficulty?
There are lots of things you can do to help your dyslexic child develop their reading skills.
It's perfectly normal for parents of children with ADHD to worry about their child's future. 
Teenager's with ADHD will require additional support and supervision from parents to avoid behaviour problems.
There is no single, easy way to treat ADHD. Treatment is made up of a mixture of medication, parent training, counselling and a special education environment.
ADHD is generally diagnosed when a number of the symptoms outlined before the age of seven and for a period of more than 6 months. 
There are many different behaviours which are characteristic of ADHD: