I wrote:
> I suspect this may be down to different options being used in
> the "zic" run that builds the zone data files, but haven't
> had time to look closer than that.
Ah ... having now refreshed my memory, the answer is that RHEL
and Apple are distributing "rearguard" tzdata files, whereas
ours are built with the main ("vanguard") data model. The
relevant difference is that the main model has the Irish rules as
being standard time in summer and negative 1 hour DST in winter.
The "rearguard" data is for parsers that cannot cope with the
idea of negative DST, so it has +1 DST in summer and a different
base GMT offset. It's alleged that this summer-time-is-standard
concept more nearly matches the way the Irish law is written;
the tzdb commentary includes
# (Note that the time in the Republic of Ireland since 1968 has been defined
# in terms of standard time being GMT+1 with a period of winter time when it
# is GMT, rather than standard time being GMT with a period of summer time
# being GMT+1.)
In short: so far as the upstream tzdb project is concerned,
we're doing the right thing, and I feel no need to change it.
If you want to match the behavior of some other software,
point --with-system-tzdata at the same zoneinfo tree that
that other software is using.
regards, tom lane