Q:

What are MIR datasets? 

What does MIR stand for? 
e.g. http://labrosa.ee.columbia.edu/millionsong/pages/additional-datasets and http://www.phenix-online.org/documentation/tutorial_mir.htm

A:

The meaning of the MIR acronym varies according to the context:

The first link you provided probably relates to this. 
The second one relates to this.