* An rí beag: This is a straightforward translation, literally "the little king." It's grammatically correct and widely understood.
* Rí beag: This is also acceptable, omitting "the." The context would usually make it clear you are referring to *a* little king.
The choice between "An rí beag" and "Rí beag" is similar to the difference between "the little king" and "a little king" in English.
There are other possibilities depending on the context and the desired emphasis, but these two are the most common and natural-sounding.