The Gardai know who murdered him, but obviously have no evidence. The interrogation didn't work to get him to confess either. Although I read somewhere that the man was a veteran of US armed forces so would have been trained in such matters I'd imagine.
One gruesome theory I read as to why there was no evidence is that the body was dismembered in a stream/river so that the blood was washed away. OMG what a horrible way to go. Sorry for the speculation, but where there's a void in information it is usually filled by such theories.
May that poor man rest in peace now. At least his family know where he is. Cold comfort indeed.