IIRC, the Aer Lingus/Aerlínte Éireann/Irish International transatlantic routes from the mid 1960’s using the 720’s and 707’s up to the early 1970’s were advertised as “Shamrock Jet Services” and used Shamrock on radio calls.
It’s possible the ‘Shamrock’ callsign spread across the EI network as more jets arrived on the European routes in the 1970’s.
JAS