Do you know if they are seasonal?
I checked Citysightseeing and you could buy tickets for that date for their HoHo Bus???
I did the DoDublin HoHo on the 6th and 7th of January 2018. I doubt they would be running in January but not April. I found their customer service very responsive via email - you might want to try contacting them directly.
Related to Easter weekend, perhaps?