They must have some specific information to have been able to get a search warrant and to be digging up a particular area. Don't know how true this, but apparently it is land which they were unable to search previously.
30 years ago, the Gardai were corrupt, no doubt about it, even on a very low level certain publicans were 'allowed' have after-hours, drink driving charges never brought, speeding fines 'lost'. it was always the influential politicians,, GAA, wealthy well connected, etc who managed to make things disappear. Murder is a whole different level though.