/usr/lib/ruby/2.5.0/irb/ is in libruby2.5 2.5.1-1ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/2.5.0/irb/cmd/
/usr/lib/ruby/2.5.0/irb/cmd/chws.rb
/usr/lib/ruby/2.5.0/irb/cmd/fork.rb
/usr/lib/ruby/2.5.0/irb/cmd/help.rb
/usr/lib/ruby/2.5.0/irb/cmd/load.rb
/usr/lib/ruby/2.5.0/irb/cmd/nop.rb
/usr/lib/ruby/2.5.0/irb/cmd/pushws.rb
/usr/lib/ruby/2.5.0/irb/cmd/subirb.rb
/usr/lib/ruby/2.5.0/irb/completion.rb
/usr/lib/ruby/2.5.0/irb/context.rb
/usr/lib/ruby/2.5.0/irb/ext/
/usr/lib/ruby/2.5.0/irb/ext/change-ws.rb
/usr/lib/ruby/2.5.0/irb/ext/history.rb
/usr/lib/ruby/2.5.0/irb/ext/loader.rb
/usr/lib/ruby/2.5.0/irb/ext/multi-irb.rb
/usr/lib/ruby/2.5.0/irb/ext/save-history.rb
/usr/lib/ruby/2.5.0/irb/ext/tracer.rb
/usr/lib/ruby/2.5.0/irb/ext/use-loader.rb
/usr/lib/ruby/2.5.0/irb/ext/workspaces.rb
/usr/lib/ruby/2.5.0/irb/extend-command.rb
/usr/lib/ruby/2.5.0/irb/frame.rb
/usr/lib/ruby/2.5.0/irb/help.rb
/usr/lib/ruby/2.5.0/irb/init.rb
/usr/lib/ruby/2.5.0/irb/input-method.rb
/usr/lib/ruby/2.5.0/irb/inspector.rb
/usr/lib/ruby/2.5.0/irb/lc/
/usr/lib/ruby/2.5.0/irb/lc/error.rb
/usr/lib/ruby/2.5.0/irb/lc/help-message
/usr/lib/ruby/2.5.0/irb/lc/ja/
/usr/lib/ruby/2.5.0/irb/lc/ja/encoding_aliases.rb
/usr/lib/ruby/2.5.0/irb/lc/ja/error.rb
/usr/lib/ruby/2.5.0/irb/lc/ja/help-message
/usr/lib/ruby/2.5.0/irb/locale.rb
/usr/lib/ruby/2.5.0/irb/magic-file.rb
/usr/lib/ruby/2.5.0/irb/notifier.rb
/usr/lib/ruby/2.5.0/irb/output-method.rb
/usr/lib/ruby/2.5.0/irb/ruby-lex.rb
/usr/lib/ruby/2.5.0/irb/ruby-token.rb
/usr/lib/ruby/2.5.0/irb/slex.rb
/usr/lib/ruby/2.5.0/irb/src_encoding.rb
/usr/lib/ruby/2.5.0/irb/version.rb
/usr/lib/ruby/2.5.0/irb/workspace.rb
/usr/lib/ruby/2.5.0/irb/ws-for-case-2.rb
/usr/lib/ruby/2.5.0/irb/xmp.rb