That wasn’t the aim of the ad but thanks to the different usage of the word “grand” in the UK and Ireland that’s what happened.
There was obviously no one Irish on the team to advise that the tagline of “it’s not great, it’s grand” was going to mean something different in Ireland!
The closest comparison would be the usage of the word fine. I’m fine/I’m grand and all the different usages depending on the tone it’s used in.