Dr Michael ParkinsonThe female part of the flower gives rise to the fruit, so to see berries you need a plant that has female parts in the flower. Most plants have both male and female parts in their flowers. An organism such as this is called an hermaphrodite. However, the English or European Holly (Ilex aquifolium) that we see in Ireland has male plants and female plants (which is termed dioecious), and you will only see berries on the female plants.
You also need the flowers on the female holly to be fertilised by pollen from the flowers of a male plant for the berries to develop. People deliberately don't grow lots of male holly because they like holly with berries on it, but you need some male plants to fertilise the female plants and in holly orchards, people plant about one male holly tree to every twenty female holly trees.
If your female holly tree is in a hedgerow then it could be a long way away from a male tree. The male tree also needs to be producing pollen at the same time that the female tree is producing its flowers and these mean that you may not get efficient pollination, and this can give poor berry production. Bees do great work in pollinating but have been in decline recently, and this could affect berry production.
Some varieties of holly, for example Nelly Stevens Holly, will produce berries without fertilisation. This is called parthenocarpy, though they are not usually as big and well developed.