Fix daylight-saving inference for Irish time

Fix daylight-saving inference for Irish time

* lisp/calendar/cal-dst.el (calendar-dst-find-data): Do not assume
that daylight-saving time has a greater UT offset than standard
time.  The reverse is true in Ireland, where standard time "IST"
is UTC +01 in summer and daylight-saving time "GMT" is UTC +00 in
winter.  The current Irish rules are given by the POSIX setting
TZ='IST-1GMT0,M10.5.0,M3.5.0/1'.