The issue is the "keyhole", an area 90W to 120W and 70N to 72N in Canada where the variation table cannot update the IRS output from true to magnetic. Some sets show HDG FAIL and require a manual switching to TRUE, others do it automatically. West coast North America to Europe typically can pass thru the keyhole.
Yes, there is one in the southern hemisphere, but few, if any, routes pass thru it. And, yes, an INS/IRS can function properly at high latitudes, in true.
GF