The Changeling is a fairly well known mythical creature, believed to have had a role in the lives of several Irish farmers. As with the Banshee, some of the Bibeanna of the Dingle Peninsula spoke of having had firsthand experience with Changelings.
A changeling is the child of a faerie (or sometimes a troll or elf) who has been swapped out for a human child. There are many different reasons people gave for fairies taking human children. Some claimed that these children would be used as servants in the faerie realm, though more state that the faeries do this simply because they want the human child, and that they child will (sometimes) be returned to their rightful parents. Some simply want to cause malice and havoc. In certain tales, the faeries covet the beauty of human children, and are more prone to taking blond children. Other tales state that the changelings are place in human care to grow up and kill or kidnap their human parents. The list goes on and on, making the changeling another very versatile figure in the folkloric landscape.
The most common idea was to throw the changeling into a fire, which would kill it and force the faeries to return the human child. Others claimed that cold steel, which burns faeries, could be put on people, and their reaction would explain whether or not they were human.
Some trace back the idea of the changeling to the actions of the cuckoo bird. When a cuckoo is ready to lay its egg, it find the nest of another bird, throws the eggs out of the nest, and lays its own in their place. The 'host' bird then raises the cuckoo as though it was their own. This is nearly identical to what people believed happened with changelings, and having observed said behavior in nature may indeed have prompted the tales of changelings.
There certainly aren't many happy facets to the stories of changelings, though they are an amazing show of the place faerie-tales played in the lives of people back in time. These were not simply myths, but realities, that they lived their lives by. This phenomenon can be explained, but is certainly most obvious in the changeling tales.