On the face of it, the GAR requirement for Ireland etc does not make sense and never did, because even the IRA are not so stupid as to not realise they can travel via e.g. France and avoid the GAR declaration.
As someone who watched the work of the IRA in a professional capacity in a past life I would like to point out that the IRA were many things but never stupid and thinking they were is a stupid comment in itself.........
My guess is that while MI6 obviously have access to all the declarations no matter who they are sent to, there is some sort of power battle (job preservation i.e. job creation) within Special Branch who are facing an increased centralisation in intelligence gathering and who are trying to hold on to at least some work.
You are having a laugh, this is not the old Eastern block you know. Those GARs end up filed in a bin not in the MI6 archive.