Well, this was actually an issue with the UN panel of experts report. I think they were granted some permission to go to Sri Lanka, but it was under such conditions that would not allow them to do independent kinds of questioning, so they were not able to visit Sri Lanka.
As you would well know, a large proportion of people have left the country. There's enough information now available to be able to conduct investigations, and to conduct interviews among the Tamil population outside the country, and there are other ways of also conducting interviews with people inside.
Obviously, it's far better for any commission of inquiry to have access to the country, but there are still ways to conduct an effective investigation.