diff --git a/test/irb/test_option.rb b/test/irb/test_option.rb
index e8622fc7b..a3a40c0b9 100644
--- a/test/irb/test_option.rb
+++ b/test/irb/test_option.rb
@@ -7,7 +7,8 @@ def test_end_of_option
       bug4117 = '[ruby-core:33574]'
       bundle_exec = ENV.key?('BUNDLE_GEMFILE') ? ['-rbundler/setup'] : []
       libdir = File.expand_path("../../lib", __dir__)
-      status = assert_in_out_err(bundle_exec + %W[-W0 -I#{libdir} -rirb -e IRB.start(__FILE__) -- -f --], "", //, [], bug4117)
+      reline_libdir = Gem.loaded_specs["reline"].full_gem_path + "/lib"
+      status = assert_in_out_err(bundle_exec + %W[-W0 -I#{libdir} -I#{reline_libdir} -rirb -e IRB.start(__FILE__) -- -f --], "", //, [], bug4117)
       assert(status.success?, bug4117)
     end
   end
diff --git a/test/irb/test_raise_exception.rb b/test/irb/test_raise_exception.rb
index 2c8136ba9..133f073f5 100644
--- a/test/irb/test_raise_exception.rb
+++ b/test/irb/test_raise_exception.rb
@@ -8,7 +8,8 @@ class RaiseExceptionTest < TestCase
     def test_raise_exception_with_nil_backtrace
       bundle_exec = ENV.key?('BUNDLE_GEMFILE') ? ['-rbundler/setup'] : []
       libdir = File.expand_path("../../lib", __dir__)
-      assert_in_out_err(bundle_exec + %W[-I#{libdir} -rirb -W0 -e IRB.start(__FILE__) -- -f --], <<-IRB, /#<Exception: foo>/, [])
+      reline_libdir = Gem.loaded_specs["reline"].full_gem_path + "/lib"
+      assert_in_out_err(bundle_exec + %W[-I#{libdir} -I#{reline_libdir} -rirb -W0 -e IRB.start(__FILE__) -- -f --], <<-IRB, /#<Exception: foo>/, [])
       raise Exception.new("foo").tap {|e| def e.backtrace; nil; end }
 IRB
     end
@@ -16,8 +17,9 @@ def test_raise_exception_with_nil_backtrace
     def test_raise_exception_with_message_exception
       bundle_exec = ENV.key?('BUNDLE_GEMFILE') ? ['-rbundler/setup'] : []
       libdir = File.expand_path("../../lib", __dir__)
+      reline_libdir = Gem.loaded_specs["reline"].full_gem_path + "/lib"
       expected = /#<Exception: foo>\nbacktraces are hidden because bar was raised when processing them/
-      assert_in_out_err(bundle_exec + %W[-I#{libdir} -rirb -W0 -e IRB.start(__FILE__) -- -f --], <<-IRB, expected, [])
+      assert_in_out_err(bundle_exec + %W[-I#{libdir} -I#{reline_libdir} -rirb -W0 -e IRB.start(__FILE__) -- -f --], <<-IRB, expected, [])
       e = Exception.new("foo")
       def e.message; raise 'bar'; end
       raise e
@@ -27,8 +29,9 @@ def e.message; raise 'bar'; end
     def test_raise_exception_with_message_inspect_exception
       bundle_exec = ENV.key?('BUNDLE_GEMFILE') ? ['-rbundler/setup'] : []
       libdir = File.expand_path("../../lib", __dir__)
+      reline_libdir = Gem.loaded_specs["reline"].full_gem_path + "/lib"
       expected = /Uninspectable exception occurred/
-      assert_in_out_err(bundle_exec + %W[-I#{libdir} -rirb -W0 -e IRB.start(__FILE__) -- -f --], <<-IRB, expected, [])
+      assert_in_out_err(bundle_exec + %W[-I#{libdir} -I#{reline_libdir} -rirb -W0 -e IRB.start(__FILE__) -- -f --], <<-IRB, expected, [])
       e = Exception.new("foo")
       def e.message; raise; end
       def e.inspect; raise; end
@@ -40,7 +43,8 @@ def test_raise_exception_with_invalid_byte_sequence
       pend if RUBY_ENGINE == 'truffleruby' || /mswin|mingw/ =~ RUBY_PLATFORM
       bundle_exec = ENV.key?('BUNDLE_GEMFILE') ? ['-rbundler/setup'] : []
       libdir = File.expand_path("../../lib", __dir__)
-      assert_in_out_err(bundle_exec + %W[-I#{libdir} -rirb -W0 -e IRB.start(__FILE__) -- -f --], <<~IRB, /A\\xF3B \(StandardError\)/, [])
+      reline_libdir = Gem.loaded_specs["reline"].full_gem_path + "/lib"
+      assert_in_out_err(bundle_exec + %W[-I#{libdir} -I#{reline_libdir} -rirb -W0 -e IRB.start(__FILE__) -- -f --], <<~IRB, /A\\xF3B \(StandardError\)/, [])
         raise StandardError, "A\\xf3B"
       IRB
     end
@@ -52,6 +56,7 @@ def test_raise_exception_with_different_encoding_containing_invalid_byte_sequenc
 
         bundle_exec = ENV.key?('BUNDLE_GEMFILE') ? ['-rbundler/setup'] : []
         libdir = File.expand_path("../../lib", __dir__)
+        reline_libdir = Gem.loaded_specs["reline"].full_gem_path + "/lib"
         File.open("#{tmpdir}/euc.rb", 'w') do |f|
           f.write(<<~EOF)
             # encoding: euc-jp
@@ -65,7 +70,7 @@ def raise_euc_with_invalid_byte_sequence
         %w(LC_MESSAGES LC_ALL LC_CTYPE LANG).each {|n| env[n] = "ja_JP.UTF-8" }
         # TruffleRuby warns when the locale does not exist
         env['TRUFFLERUBYOPT'] = "#{ENV['TRUFFLERUBYOPT']} --log.level=SEVERE" if RUBY_ENGINE == 'truffleruby'
-        args = [env] + bundle_exec + %W[-I#{libdir} -rirb -C #{tmpdir} -W0 -e IRB.start(__FILE__) -- -f --]
+        args = [env] + bundle_exec + %W[-I#{libdir} -I#{reline_libdir} -rirb -C #{tmpdir} -W0 -e IRB.start(__FILE__) -- -f --]
         error = /raise_euc_with_invalid_byte_sequence': あ\\xFF \(RuntimeError\)/
         assert_in_out_err(args, <<~IRB, error, [], encoding: "UTF-8")
           require_relative 'euc'
diff --git a/test/irb/test_ruby_lex.rb b/test/irb/test_ruby_lex.rb
index 8911a6cd4..9392b49fa 100644
--- a/test/irb/test_ruby_lex.rb
+++ b/test/irb/test_ruby_lex.rb
@@ -201,7 +201,8 @@ def test_assignment_expression_with_local_variable
 
     def test_initialising_the_old_top_level_ruby_lex
       libdir = File.expand_path("../../lib", __dir__)
-      assert_in_out_err(["-I#{libdir}", "--disable-gems", "-W:deprecated"], <<~RUBY, [], /warning: constant ::RubyLex is deprecated/)
+      reline_libdir = Gem.loaded_specs["reline"].full_gem_path + "/lib"
+      assert_in_out_err(["-I#{libdir}", "-I#{reline_libdir}", "--disable-gems", "-W:deprecated"], <<~RUBY, [], /warning: constant ::RubyLex is deprecated/)
         require "irb"
         ::RubyLex.new(nil)
       RUBY
diff --git a/test/irb/test_workspace.rb b/test/irb/test_workspace.rb
index 62166abfd..99ade80f6 100644
--- a/test/irb/test_workspace.rb
+++ b/test/irb/test_workspace.rb
@@ -84,12 +84,13 @@ def test_toplevel_binding_local_variables
       bug17623 = '[ruby-core:102468]'
       bundle_exec = ENV.key?('BUNDLE_GEMFILE') ? ['-rbundler/setup'] : []
       top_srcdir = "#{__dir__}/../.."
+      reline_libdir = Gem.loaded_specs["reline"].full_gem_path + "/lib"
       irb_path = nil
       %w[exe libexec].find do |dir|
         irb_path = "#{top_srcdir}/#{dir}/irb"
         File.exist?(irb_path)
       end or omit 'irb command not found'
-      assert_in_out_err(bundle_exec + ['-W0', "-I#{top_srcdir}/lib", "-C#{top_srcdir}", '-e', <<~RUBY, '--', '-f', '--'], 'binding.local_variables', /\[:_\]/, [], bug17623)
+      assert_in_out_err(bundle_exec + ['-W0', "-I#{top_srcdir}/lib", "-I#{reline_libdir}", "-C#{top_srcdir}", '-e', <<~RUBY, '--', '-f', '--'], 'binding.local_variables', /\[:_\]/, [], bug17623)
         version = 'xyz' # typical rubygems loading file
         load('#{irb_path}')
       RUBY