IRPQLSRV is, as you suspected, almost entirely used only for satisfying requests from the Web Interface. It's the process that receives and combines queries from Web UI dashboards, sends those requests to the Prognosis server via the irnetrtr process, and then sends the data back for display.
If you aren't using the web interface though you don't need to have it running. You can turn it off by adding the following line to the Prognosis configuration, and starting the configuration with the change in place:
SET RUN ( IRPQLSRV.EXE, N )
(If you already have a line in the configuration with the parameter set to 'Y', just change it to 'N'. If you don't have such a line, just add it in as I've shown here).
Good point. So perhaps it's more accurate to say "If you do not have any custom solutions, irpqlsrv is used only for the Web UI. If you do have a custom solution, it may rely on irpqlsrv and you should check in with your IR account contact before disabling it" :-)