A terrible omen of death, the Irish fetch spells bad news if it appears. Not to the person it appears to, but for the subject it resembles.
Like skinwalkers of Native American folklore, the fetch is a shapeshifter. As a result, one may not realize they're in the presence of a fetch, or even interacting with one.
Very similar to the idea of the Doppelganger, the fetch spells doom for whomever it decides to impersonate. The person who ends up merely interacting with the fetch simply becomes the bearer of bad news, that is, if they realized they've encountered a fetch.
The next time you find yourself walking along in Ireland, and you encounter somebody you know, make sure you attempt to speak to them. A lack of response means you likely came across a fetch, and your acquaintence may be in grave danger.