Investors are looking at Irish banks’ enormous first-half losses, announced this week, and trying to figure out what’s going on.  There are two possibilities. Either AIB and Bank of Ireland are two of the most cautiously-managed banks in the world, or something particularly nasty is lurking in Irish loan books. The issue is over write-downs. AIB and Bank of Ireland have written down the value of their assets by an enormous amount by banking industry standards. AIB has written down the value of its loan book by €1.2 billion in the first half. Bank of Ireland has written it down…