"Mag-Var" tables are included in the Flight Management Computer software. These tables are stored in memory and include predicted changes in the magnetic variations between Latitudes 82 north and 82 south. The tables are periodically revised to ensure they match the latest magnetic variation data available. These data are never completely up to date - its a never ending task like painting the Sydney Harbour Bridge - but its the best that can be done. The IRS itself doesn't bother with the magnetic information - it carries on navigating by reference to True North. The magnetic variation data converts 'true' to 'magnetic' headings for use in the external environment, which persists in using magnetic references for runway headings, Navaid alignment and the like. I suspect the continued use of the increasingly unstable magnetic reference is intended to keep the navaid calibration industry in employment. Or simple old fashioned "Tradition"