A few posts back I wrote about the secret fairy-paths of the air, and how a person might inadvertently get whisked away being caught up in the fairy parade. While the most well known folklore regarding this phenomenon warns of particular days such as the equinoxes as being times to avoid fairy mounds, there are plenty of accounts of fairy parades throughout the whole year.
Perhaps what's difficult here is being definite about what a parade is, and what the good people are enacting or carrying out, themselves. For example, a long held tradition is that if a person was going to build a house they would place some stones upon the site before building began. If they returned the next day and the stones had been moved or shattered this indicated that the planned house was situated on a fairy path and would have to be built somewhere else. If a person went ahead and built upon the site then not only would they suffer terrible luck, but the physical structure of the building would often become damaged, with entire walls coming down in some instances. Needless to say, there were various ways a person might attempt to remedy this. In some cases a fairy doctor might be called. This might be a solution if animals on the land were considered to have been elf-shot or injured with fairy-darts. (Source)
However, it was also the case that a person might just have to amend the structure of the building in order to accommodate the fairy path. This could mean a wall being knocked down or an out-building housing livestock having to be moved entirely. (Source)
The question of why this occurred is actually more complicated than you might think. One usual answer is that the fairies were displeased and expressed their anger by physical damage to the obstacle in their way. By causing sickness to the occupants and livestock it was believed that the good people deliberately sought out a way to inflict ill-will upon humans and animals.
However, another argument here ties into the belief that the dead often accompanied fairies on these parades. The dead themselves were often divided into different classes or groups depending on how they had died themselves. This might result in a benign encounter if they had died well or peacefully, but on the other hand, encountering the dead who had died violently might have a different outcome.
Writing in The Return of the Dead: Ghosts, Ancestors, and the Transparent Veil of the Pagan Mind, Professor Claude Lecouteux tells us that in Northern Europe it was believed that to fade away after a long life was seen to be a blessing, but to die prematurely was interpreted as being a curse. Lecouteux describes the various types of dead including ghosts, phantoms, revenants and larva (a Roman word meaning a deceased individual who has been denied eternal rest) and how being haunted by these spirits might have differing outcomes. Today, ghosts and spirits, much like fairies, have changed from how they were understood in more ancient times. Lecouteux describes how, following the 16th century, apparitions began to become more simple literary motifs, and finally tropes in horror films. It is a mistake to see ghosts this way if we are going to draw upon a legacy of folklore and myths, though.
Coming back to the fairy parade, which we are told in our folklore does indeed include the dead, we might understand the dangers of getting caught in this troop on a deeper level. In ancient times, the dead were often considered to be impure and dangerous, even if they could not help this state of affairs. They had a contaminating effect upon the living who encountered them even if they could be placated by certain rituals and acknowledgements. We can see how this connects to the paying of respects to ancestors and sacred places, of course, but also another way of understanding the taboo nature of certain places and times.
You might notice a parallel to cilliní in some ways: the dead who walked with the fairies may have died in unfavourable circumstances. Perhaps prayers were not said for them or they died forgotten and ashamed. With these different layers of context, then, perhaps it makes it easier to see just why avoiding a fairy-path was so important. It is not only a matter of avoiding bad luck and becoming entangled within a damaging energy, but, potentially also a way to redeem or repair a lost soul. These souls are understood to be bound somehow, even if the ultimate reasons why fairies took these souls with them is still not fully understood. (C.) David Halpin.