You might find something at the On-line Paper Company and there’s a product called Inkjet postcards by a company called Sigel here. Why not just create a postcard template and then print on the back of 6×4 photo paper before printing your image on the front? Or have you ruled this out for some reason (wrong surface on the back, etc.)?
You can also pick them up in different sizes, quanitiy, quality and colours in the Art and Hobby shop in Jervis and other places – not sure if they’re in Cork though ??