DHL is owned bu Deutche Post, and all DP parcels are delivered in Ireland by An Post.
I have some experience of customs both here and Germany as a result of some audits of postal procedures. Items originating in Germany should have no duty or vat levied on them on arrival in Ireland, or any other EU country. However, if the items are held in bond in Germany, and then dispatched here, they are only arriving in the EU when they hit Ireland. In this case, vat and duty would apply. Differences in charges due could be explained by different freight charges or exchange rates ( if items billed in a non euro currency). If you ask me that's whats happening. It's one of the big buyer beware issues when buying from the web. It does not naturally follow that if the seller is in Europe, that the goods are too. Many are duped buying off eBay and other sites.
You will be lucky to get a reply from revenue at all. If the item was delivered by An Post, have you tried phoning revenue officers in portlaoise mail centre or dublin mail centre.