LONDONDERRY—Ireland on Sunday called for Britain to ensure justice for the families of 13 peaceful protesters shot dead by its soldiers on “Bloody Sunday” in 1972 as thousands marked the 50th anniversary of one of the defining days of the Northern Ireland conflict. The British government in 2010 apologised for the “unjustified and unjustifiable” killings of 13 Catholic civil rights protesters by British soldiers in Londonderry on Jan. 30, 1972—and of a 14th who died later of his wounds. But none of those responsible for the shootings have been convicted and last July British prosecutors announced that the only British soldier charged with murder will not face trial—a decision that is being challenged by relatives. “There should be a route to justice,” Irish Foreign Minister Simon Coveney told state broadcaster RTE after laying a wreath and meeting with relatives of the victims. “As somebody said, our children were buried 50 …