If I'm reading your code correctly, then your TBX file lives at the top-level inside of your 'Install' directory. When you build an add-in, the files inside of 'Install' are all placed into a GUID-based directory within your users' home directory, and any paths should be relative to this. You can read more about using files from the 'Install' directory in the documentation, under the heading "File and folder structure".
So, the path to the toolbox should be something like:
toolPath = os.path.join(relPath, "getIR.tbx")
One way you can test this is by changing your code to just pop up a message box instead of executing the tool:
def onclick(self):
pythonaddins.MessageBox("toolPath is set to {}.".format(toolPath), "Expected Path")
cheers,
Shaun