Shamrock & SACO - well you both answered it partly correctly and the correct answer is a combination of your answers.
It is the Halseylec (not Halselec), built during 1954 and completed 1955 for its first race at the British Empire Trophy on 2 April 1955.
It was built for Eric Brandon by his mechanic Michael 'Ginger' Devlin and was fitted with a Climax FWA engine.
The car was registered WPK 6.
A second car was built in 1956 (I'll keep that as a separate puzzle).
The name Halseylec stems from the telegraphic address of Eric Brandon's family electrical wholesaling business.
I think it's fair to award you a point each in this case.