Holly trees not producing berries

I live in south Co Wexford, about 2 miles from the sea. I have 3 or 4 different holly types in my garden, with a total of about 40 plants. They range from 3-10 years old but they have never produced any berries. They are very healthy looking plants and all are growing well. I understand that I need male and female types but whenever I ask in a garden centre no-one can ever tell me whether the plants are male of female. Any ideas?