Conference presentation of a weak paper to an expert audience.
Answer inspired by @Roland's comment.

First of all, have a little faith in yourself, your supervisor seems to believe your work is good enough and so should you.

Otherwise, if you really do have some doubts about the relevance of the paper you can stress the fact that these results seem to be unknown in the community.

Raise awareness on the fact that your project is interesting and that some work and results have already been made, yet seem to have passed under the radar of many.