You will need a visa.
From the EU Immigration Website:
Moving between EU countries during my long-term stay – more than three months
When you stay in an EU country for a long stay, usually for more than three months, you will generally be issued with a national long-stay visa and/or a residence permit.
If your long-stay visa or permit has been issued by a Schengen area country, you can travel to another Schengen area country for three months per six-month period. [...]
Germany is part of Schengen. If you have a long-term visa in one Schengen country, you can freely visit the other 25 for up to three months without a visa.
The UK and Ireland are not part of Schengen, and thus excempt from this rule.