Paul:
Over here most of the rural areas were either still manually switched or on old ericsson ARK crossbar exchanges. Very few low population areas had SxS switching.
1979 - 1984/85 all rural areas went to digital switching as small and larger Alcatel E10B exchanges were rolled out. They were mostly 0XX-YXXXX. 5 digit numbering was sufficient in most of these areas. Dublin (01) and Cork (021) only had 6-digit numbering! Dublin migrated to 7-digits in the late 80s and Cork more recently.
Mostly the numbers in 5-digit areas start with 5/4 or 2
As the demand for numbers, due to various telephone companies appearing and the requirment for ISDN Direct inward dial most of the old 5-digit areas were shifted to 7 to free up more structured spac.. They usually just added a 2 digit prefix to the old number. In other areas old area codes were merged to simplify the system.
