There are a series of different UN rapporteurs on various aspects; I think there are dozens of them. The Sri Lankan government did allow the High Commissioner for Human Rights to visit in August of this past year. She then presented an oral statement to the Human Rights Council, in September, in which she was strongly critical about what she saw on her visit, including, she mentioned, the harassment of witnesses and people who had talked to her, by the military and police. That was while she was still in the country, which was quite shocking to her. She referred to what she called the “growing authoritarian tendencies”, or direction in which the government is moving.
The government let her in, I think, to be able to say that they're cooperating with the office of the high commissioner. But crucially, many other special rapporteurs, most especially on extrajudicial killings, on torture, on transitional justice, and the UN Working Group on Enforced or Involuntary Disappearances, have been asking to come, many of them literally for years, and the Sri Lankan government has refused. Unfortunately, there's no way that they can go in if they don't have the invitation of the government.
I agree that the international system doesn't have all of the tools it really needs, unfortunately. And when they are there, it often takes a long time to get the machine up and running.