1977 was a pivotal year Noel, Lima changed their HO scale to OO
Hornby had not manufactured any "Irish" models before the OO range was launched
Lexie Keady hooked up with Lima in 1977 when the Wrenn Brothers ceased their agency for Lima N Gauge and I reckon it was then that the HO Irish stuff was commissioned, just before they turned to OO gauge manufacturing
So 77 would be the earliest for your polaroid,
Hornby saw the Lima Irish bits some time later and launched their Hymek sets which lasted from mid 78 till 81 if I remember correctly