Most have no interest in racing but nearly every compatriot of that time remembers Shergar. When Prince Karim Aga Khan, the billionaire and spiritual leader, passed away aged 88 earlier this week, it closed another chapter in a story without an ending, one of which the Cohen brothers would be proud. There, too, was something uniquely Irish about its character. Back in Co Meath, in the year 1981, an 11-year-young Johnny Murtagh – with no interest in the sport – watched on television as this astonishing bay demolished his Derby field by ten lengths. Murtagh, now the Aga Khan’s main…