Not only are they not poisonous but they are actually edible.

Fruit - raw or cooked. An acid flavour, but it is rather nice raw especially when added to muesli or porridge[K]. Unfortunately, there is relatively little flesh and a lot of seeds[K]. The ovoid fruit is about 9mm long[200], it ripens in spring and crops can be quite good if the plant is in a sheltered position[K]. The fruit is produced in large clusters and so is easy to harvest[K].
Quoted from Plants for a Future.

A related species, Mahonia aquifolium ("Oregon Grape") was eaten by settlers in North America and was formerly a food crop of native peoples.