ir_solution is the filename extension of the files used to save Foldit solutions.
The format of an ir_solution file is not known. Tags like "SOLN" and "META" may be human-readable, along with a solution name and description. The rest of the file appears to be binary data.
Logically, an ir_solution file should contain the same information as a PDB file, which would include the spatial coordinates of each atom in the protein.
In practice, ir_solution files vary considerably in size, so it's not clear that they all contain the same type of information. For example, an ir_solution for a manual save might be over 200 KB, while the ir_solution for a quicksave might be close to 50 KB. ("KB" means kilobyte.)
Examples of ir_solution filenames:
See Foldit file and directory structure for more.