Ireland's OWN: Celtic Symbols

The Web

To the Celts, the spider's web symbolised the web that held all life together. For Egyptians and Greeks it stood for fate.
The spider itself represents the Great Mother in her devouring aspect.
A spider's web can be used to scry in the same way as a crystal ball for those who practice divination.
See also Spiders as Companions in the Home.

Sources:

Cunningham, S and Harrington, D. The Magical Household: Spells & Rituals for the Home. Llewellyn Worldwide, 2003.
Fontana, D. The Secret Language of Symbols. Chronicle Books, 1993.