Intergaph SmartSketch diagramming and technical drawing software supports several CAD formats, including the DWG, in which it can export its own IGR drawing designs. It doesn't look like that any other program supports the IGR format, so it is probably your only choice for IGR to DWG conversion.
Convert SmartSketch drawing design file to AutoCAD drawing.
We found one software available in the database for this conversion.
Microsoft Windows software - convert igr to dwg on Windows
Technical drawing program from Intergraph
Convert from igr file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.