Is it at all possible that in the knowledge that certain online retailers will compensate purchasers for missing packages, those items with the retailers name emblazoned on the packaging have now become targets for thieves?
OP if you have a tracking number from the retailer, you can track the package through all stages of the delivery process, including the last known mail centre before the delivery was signed for. You can send this information to An Post and have them check their systems.
I know how frustrating it can be when stuff goes missing like this.