You will have service in Germany. As you have an iPhone you will need to try a couple of little tricks to get connected.
First of all try disabling 3G/4G on your handset and try connecting to a network.
If above fails, go to the Settings/Phone/SIM Applications and select When Abroad/Network Selection/Manual/International - Roaming profile. Then search manually for a network and try to connect.
One of those will let you connect to a German network.
Cost wise you have a couple of options.
You can buy any of the EU roaming add ons.
EU Roaming VOICE - €3.50
Ten minutes of outbound calls within the EU.
Unlimited inbound calls within the EU.
Free to receive calls.
EU Roaming text - €2
Twenty outbound texts within the EU.
Unlimited inbound texts within the EU.
Or if you will only be using your phone in emergencies then just top up with some credit. You will then be charged for each call you make or receive and each text you send. See the costs here http://www.48months.ie/membership-plans/going-abroad under the "Standard EU roaming charges
If you need anything else let me know ~ Damian from 48
Edit: post crossed with Damian's...
Are you using an iPhone?
If so then you might need to be aware of this:
The vast majority if not all problems reported with roaming abroad seem to be with iPhones. I use Android and have never had problems roaming in the EU.
If in doubt maybe ask 48 support in advance?
Hope this helps.
OK - thanks. I understand now.
That's the trick that works around the stupid way that Apple do things with iPhones and roaming.
Yet another way that they make things unnecessarily complicated for users.
Others include the need to activate iPhones/iMessage/FaceTime etc. on networks, the fact that people no longer get text messages when they stop using iMessage unless they know to detach their account from it etc.