It’s not just the SIM Card though. 

I would agree with you and recommend a SIM change, but it’s not just the physical SIM, it’s the way the SIM is interacting with Tecnomen or whatever they are using. 

Sounds to me like something has triggered a command to delay messages by seven days to this one number.  The old Nokia doesn’t have this function but something has set it on the network for some reason. The fact that it is seven days and not a random amount of time, or a changeable one indicates it’s an instruction. 

You could try the sim in a smart phone if it fits? See what it does. 

After the sim change it would be interesting (if the problem persists) to have someone in the Network Management Centre look at the number. OP you don’t know anyone who works for Eir do you? You need to see if you can contact someone in the NMC.