Possible modern equivalents:
* "Boy": This is the most common and direct replacement.
* "Lad": This is a slightly more affectionate and informal alternative.
* "Young man": This is a more formal and respectful option.
* "Hey you": This is a casual and somewhat rude option.
* "Listen here": This emphasizes the speaker's authority and demands attention.
The best way to translate "sirrah" depends on the specific context and the tone you want to convey.