Germany has had a positive balance of trade (a trade surplus, i.e. it exported a lot more than it imported). If Germany had had its own currency (e.g. DM), importing nations would have had to buy DM in order to exchange them for German goods. The demand for DM would have pushed up the price of DM; this would have made importing goods from Germany more expensive.

But with the Euro, other countries in the Eurozone which have had a negative balance of trade (a trade deficit) balanced out Germany, preventing the Euro from appreciating too much. This helped Germany's export sector, helping keep them competitive.

The other issue with the Euro is that Germany's economy was already adjusted for low interest rates; but the introduction of the Euro meant low interest rates in countries which had not historically had them.

Ireland didn't have a big government deficit problem; Ireland's problem was a property bubble driven by reduced rates on mortgages. It only turned into governmental debt when the government guaranteed bank debt, having been convinced by the banking-political cabal in the middle of the 2008 crisis that the bank problem was liquidity, not solvency; but with banks holding assets formed from inflated house prices, they didn't have as much capital as they thought.