If you just want to suppress long output once in a while, use ;0, like:
a = [*1..10000];0
# => 0
If you want to suppress it generally, use the ~/.irbrc file. The IRB.conf[:INSPECT_MODE] and IRB.conf[:PROMPT][your_prompt][:RETURN] control what is returned. You can figure out what your_prompt is by checking IRB.conf[:PROMPT_MODE]
Example:
IRB.conf[:PROMPT][:DEFAULT][:RETURN] = "" # suppress return value completely
You’ll need to restart irb after changing the value.