Although the following custom is considered romantic, I must admit that I've always found it slightly eerie and disturbing. There are a few variations, but, basically, it goes like this:
A young girl who wants to discover who she will marry stands before a mirror on Halloween night. She then lights a candle and, depending on the version, either she will see her future husband in the mirror, or this would be achieved by having to fulfil a task such as eating or peeling an apple. The peeling was attempted in order to achieve one single cut with no break in the apple skin. When the peel fell to the ground, the initials of the future loved one was said to reveal itself in the shape of the skin.
There is also a version of this custom where the girl placed the apple next to the mirror by candlelight at midnight, but instead of peeling it she combs her hair. Two examples from Irish folklore here and here.
Personally, I have always wondered about the magical connotations of this custom: the future husband being seen in the mirror seemed to imply that fate could not be interfered with and that a persons life was predestined.
Another metaphysical aspect here is that the future self is also sometimes seen in the mirror, as if verifying the spell. This links us to the idea of mirrors being used for prophecy and divination, which is probably another post onto itself.
This phenomenon corroborates the belief that time ran differently in the Otherworld or Mirrorworld, which we have come across a few times in Irish folklore on this page. The reversal or reflection motif also has many non-Samhain links to fairy lore, such as turning clothes inside out or walking backwards to break a spell of being pixie-led, for example.
Another common fairy account tells us that a person trapped in the fairy Otherworld can sometimes be seen in the reflection of glass, such as the famous example of when a new bride was able to give her husband instructions for how to rescue her from her fairy captors when he glimpsed her image upon his window. What is interesting in these examples is not only the reflection giving access to the fairy world but also how a mirrored surface seems to allow back and forth communication between the physical and fairy world in some instances.
This is also seen in the association between reflections in water and the appearance of fairies. Claude Lecouteux, in Demons and Spirits of the Land, tells us that, "During the fifteenth century it was almost proverbial to say something was "as naked as a fairy coming out of the water."
It is no coincidence that mirrors have such associations with fairies at Samhain considering how in Irish lore this time of the year is traditionally a period where fairies and the dead seem to move between realms more easily. Then again, reflective surfaces being considered conduits to connect us to the realm of the dead is also very common in other areas of folklore and superstition.
One custom you may know is that of covering mirrors in a house after a person dies. A reason given for this is that the soul of the deceased may become trapped in the mirror. This idea turns up regularly in horror films and tales but, really, there is a protective and healing aspect to the ritual for both the deceased and household which is sometimes overlooked.
A darker twist on this custom is that by seeing yourself in the mirror of someone who has died you may inadvertently bring the dead, and your death, closer. An Irish folklore tale of a soul trapped in a mirror can be read here.
There is also a belief that scrying at this time of the year gives easier access to spirits and allows a person to communicate with gods, ancestors and the Otherworld. Scrying can be carried out in a number of ways, but the basic idea is that a person stares into a mirror, preferably a black reflective surface, until they fall into an altered state and the barriers between perception and the Otherworld dissipate.
As usual with such customs, there is also an opposing use or purpose. For example, Kevin Turner, writing in Sky Shamans of Mongolia, tells us that within Asian shamanism, mirrors are used to protect the journeying shaman, who will hang, or tie, small metal mirrors to their bodies in order to scare away vindictive spirits.
In Nepalese shamanism these objects are called Aina (In Tibetan, these are called Melong) and can also draw in positive forces to protect the shaman. In this context, we see a connection to European fairy tradition in that the being emerges through the reflective surface, much like the opening of a portal. An interesting aside here is that when the mirror is positively charged it can reflect healing towards the shaman's patient.
Finally, returning to Ireland, Biddy Early was said to use her blue bottle in similar ways. The reflections, within the substance inside the glass, were believed to allow her to view future events, communicate with fairies, and discover supernatural afflictions which had been placed maliciously upon people and animals. In this sense, the bottle was less a portal to travel through, and more a window to allow Biddy access information and wisdom. That said, there is probably a good argument to be made that Biddy's consciousness travelled to the fairy realm while using the bottle, so perhaps there are deeper implications here. (C.) David Halpin. Image: If Only I Could by Jakub Rozalski.