By Clodagh Kilcoyne LONDONDERRY (Reuters) -Ireland on Sunday called on 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 the Northern Irish city of Londonderry on Jan. 30, 1972 – and of a 14th who died later of his wounds. None of those responsible for the shootings has b…