One of them was on the phone at 2.45pm when Michaela McAreavey was supposed to have been strangled, so it seems in the jury's mind that proved beyond a reasonable doubt that he wasn't involved. As for the other guy, they believed his story that his signed confession was a fabrication. There was no DNA evidence from either of the accused in the hotel room. It seems most likely that the cops fucked up the investigation more than anything.
It's known as the least corrupt legal system in the continent of Africa and is predominantly based on the common law, the same as this country, but then everyone says the system in this country is corrupt too, so you can't win