diff --git a/gio/tests/meson.build b/gio/tests/meson.build
index ba04916c3..26091b6e7 100644
--- a/gio/tests/meson.build
+++ b/gio/tests/meson.build
@@ -1175,6 +1175,7 @@ endforeach
 
 python_test_env = test_env
 python_test_env.prepend('PYTHONPATH', python_test_libraries_path)
+python_test_env.prepend('PYTHONPATH', python_test_libraries_built)
 
 foreach test_name, extra_args : python_tests
   depends = [extra_args.get('depends', [])]
diff --git a/girepository/girparser.c b/girepository/girparser.c
index 0cf341533..63143718d 100644
--- a/girepository/girparser.c
+++ b/girepository/girparser.c
@@ -303,6 +303,8 @@ static GMarkupParser firstpass_parser =
   NULL,
 };
 
+/* If you change this search order, gobject-introspection.git and
+ * tests/installed/glibconfig.py.in will probably also need updating */
 static char *
 locate_gir (GIIrParser *parser,
             const char *girname)
diff --git a/girepository/meson.build b/girepository/meson.build
index db14088b9..3010099aa 100644
--- a/girepository/meson.build
+++ b/girepository/meson.build
@@ -29,7 +29,10 @@ else
   gir_dir_pc_prefix = join_paths('${prefix}', gir_dir_prefix)
 endif
 
-glib_girdir = get_option('prefix') / gir_dir_prefix / 'gir-1.0'
+# This is effectively the GIR XML format major version
+gir_suffix = 'gir-1.0'
+
+glib_girdir = get_option('prefix') / gir_dir_prefix / gir_suffix
 
 gir_includedir = glib_includedir / 'girepository'
 
@@ -81,7 +84,7 @@ install_headers(girepo_headers + girepo_ffi_headers, install_dir: gir_includedir
 gir_c_args = [
   '-DGI_COMPILATION',
   '-DG_LOG_DOMAIN="GLib-GIRepository"',
-  '-DGIR_SUFFIX="gir-1.0"',
+  '-DGIR_SUFFIX="@0@"'.format(gir_suffix),
   '-DGIR_DIR="@0@"'.format(glib_girdir),
   '-DGOBJECT_INTROSPECTION_LIBDIR="@0@"'.format(glib_libdir),
   '-DGOBJECT_INTROSPECTION_DATADIR="@0@"'.format(glib_datadir),
@@ -227,7 +230,7 @@ executable('gi-dump-types',
 
 pkgconfig_variables = [
   'gidatadir=${datadir}/gobject-introspection-1.0',
-  'girdir=' + gir_dir_pc_prefix / 'gir-1.0',
+  'girdir=' + gir_dir_pc_prefix / gir_suffix,
   'typelibdir=${libdir}/girepository-1.0',
   'gi_compile_repository=' + pkgconfig_multiarch_bindir / 'gi-compile-repository'
 ]
diff --git a/girepository/tests/gi-compile-repository.py b/girepository/tests/gi-compile-repository.py
index a0b2bcf7e..8718796b5 100644
--- a/girepository/tests/gi-compile-repository.py
+++ b/girepository/tests/gi-compile-repository.py
@@ -20,11 +20,12 @@
 # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
 # MA  02110-1301  USA
 
-""" Integration tests for gi-compile-repository. """
+"""Integration tests for gi-compile-repository."""
 
 import os
 import unittest
 
+import glibconfig
 import taptestrunner
 import testprogramrunner
 
@@ -39,20 +40,9 @@ class TestGICompileRepositoryBase(testprogramrunner.TestProgramRunner):
     def setUpClass(cls):
         super().setUpClass()
 
-        if "G_TEST_BUILDDIR" in os.environ:
-            cls._gir_path = os.path.join(
-                os.environ["G_TEST_BUILDDIR"], "..", "introspection"
-            )
-        else:
-            cls._gir_path = os.path.join(
-                os.path.dirname(os.path.realpath(__file__)),
-                "..",
-                "..",
-                "..",
-                "share",
-                "gir-1.0",
-            )
-        print(f"gir path set to {cls._gir_path}")
+        # list of pathlib.Path
+        cls._gir_paths = glibconfig.GIR_XML_SEARCH_PATHS
+        print(f"gir path set to {cls._gir_paths}")
 
 
 class TestGICompileRepository(TestGICompileRepositoryBase):
@@ -73,13 +63,20 @@ class TestGICompileRepositoryForGLib(TestGICompileRepositoryBase):
     GIR_NAME = "GLib-2.0"
 
     def runTestProgram(self, *args, **kwargs):
-        gir_file = os.path.join(self._gir_path, f"{self.GIR_NAME}.gir")
-        self.assertTrue(os.path.exists(gir_file))
-        argv = [gir_file]
+        for d in self._gir_paths:
+            gir_file = d / f"{self.GIR_NAME}.gir"
+
+            if gir_file.exists():
+                break
+        else:
+            self.fail(f"Could not find {self.GIR_NAME}.gir in {self._gir_paths}")
+
+        argv = [str(gir_file)]
         argv.extend(*args)
 
         if self.GIR_NAME != "GLib-2.0":
-            argv.extend(["--includedir", self._gir_path])
+            for d in self._gir_paths:
+                argv.extend(["--includedir", str(d)])
 
         return super().runTestProgram(argv, **kwargs)
 
diff --git a/girepository/tests/meson.build b/girepository/tests/meson.build
index fb3b5b031..ab5e9f748 100644
--- a/girepository/tests/meson.build
+++ b/girepository/tests/meson.build
@@ -183,6 +183,7 @@ endif
 
 python_test_env = test_env
 python_test_env.prepend('PYTHONPATH', python_test_libraries_path)
+python_test_env.prepend('PYTHONPATH', python_test_libraries_built)
 
 foreach test_name, extra_args : python_tests
   depends = [extra_args.get('depends', [])]
diff --git a/glib/tests/meson.build b/glib/tests/meson.build
index 5734001e2..b662c3713 100644
--- a/glib/tests/meson.build
+++ b/glib/tests/meson.build
@@ -514,6 +514,7 @@ endif
 
 python_test_env = test_env
 python_test_env.prepend('PYTHONPATH', python_test_libraries_path)
+python_test_env.prepend('PYTHONPATH', python_test_libraries_built)
 
 foreach test_name, extra_args : python_tests
   depends = [extra_args.get('depends', [])]
diff --git a/gobject/tests/meson.build b/gobject/tests/meson.build
index 933461f5a..268dd5dd9 100644
--- a/gobject/tests/meson.build
+++ b/gobject/tests/meson.build
@@ -234,6 +234,7 @@ endforeach
 
 python_test_env = test_env
 python_test_env.prepend('PYTHONPATH', python_test_libraries_path)
+python_test_env.prepend('PYTHONPATH', python_test_libraries_built)
 
 foreach test_name, extra_args : python_tests
   depends = [extra_args.get('depends', [])]
diff --git a/meson.build b/meson.build
index a8bcadc8d..af344634f 100644
--- a/meson.build
+++ b/meson.build
@@ -2500,6 +2500,7 @@ assert(fs.exists(python_test_libraries_path))
 if not python_version.version_compare(python_version_req)
   error('Requires Python @0@, @1@ found.'.format(python_version_req, python_version))
 endif
+python_test_libraries_built = meson.project_build_root() / 'tests' / 'lib'
 
 # Determine which user environment-dependent files that we want to install
 bash = find_program('bash', required : false)
diff --git a/tests/black.sh b/tests/black.sh
index 37473bef5..2e214900e 100755
--- a/tests/black.sh
+++ b/tests/black.sh
@@ -16,7 +16,7 @@ export TEST_REQUIRES_TOOLS="black git"
 
 run_lint () {
     # shellcheck disable=SC2046
-    black --diff --check $(git ls-files '*.py')
+    black --diff --check $(git ls-files '*.py' 'tests/lib/*.py.in')
 }
 
 # shellcheck source=tests/lint-common.sh
diff --git a/tests/flake8.sh b/tests/flake8.sh
index 10a0a6ffd..627a2aed7 100755
--- a/tests/flake8.sh
+++ b/tests/flake8.sh
@@ -21,7 +21,7 @@ run_lint () {
     # shellcheck disable=SC2046
     flake8 \
         --max-line-length=88 --ignore="$formatting_warnings" \
-        $(git ls-files '*.py')
+        $(git ls-files '*.py' 'tests/lib/*.py.in')
 }
 
 # shellcheck source=tests/lint-common.sh
diff --git a/tests/lib/glibconfig.py.in b/tests/lib/glibconfig.py.in
new file mode 100644
index 000000000..7962fbab3
--- /dev/null
+++ b/tests/lib/glibconfig.py.in
@@ -0,0 +1,29 @@
+# Copyright 2025 Simon McVittie
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+import os
+from pathlib import Path
+
+
+if "G_TEST_BUILDDIR" in os.environ:
+    # During build-time testing, we search the build tree.
+    # We can't use the value of G_TEST_BUILDDIR here because we don't
+    # know how deeply nested the current build directory is.
+    project_build_root = Path(__file__).resolve().parent.parent.parent
+
+    # Where to find GLib-2.0.gir, etc. as a list of Path, highest priority first
+    GIR_XML_SEARCH_PATHS = [
+        project_build_root / "girepository" / "introspection",
+    ]
+
+else:
+    # During as-installed testing, we search the final paths.
+
+    # This is a subset of the search path from girparser.c locate_gir(),
+    # with only the directories that are part of GLib's prefix,
+    # excluding XDG_DATA_HOME, XDG_DATA_DIRS and the hard-coded
+    # /usr/share fallback
+    GIR_XML_SEARCH_PATHS = [
+        Path(r"@glib_girdir@"),
+        Path(r"@glib_datadir@") / r"@gir_suffix@",
+    ]
diff --git a/tests/lib/meson.build b/tests/lib/meson.build
new file mode 100644
index 000000000..25990a481
--- /dev/null
+++ b/tests/lib/meson.build
@@ -0,0 +1,26 @@
+# Copyright 2024 Collabora Ltd.
+# Copyright 2025 Simon McVittie
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+install_data(
+  files(
+    'taptestrunner.py',
+    'testprogramrunner.py',
+  ),
+  install_dir: installed_tests_execdir,
+  install_tag: 'tests',
+)
+
+tests_conf = configuration_data()
+tests_conf.set('gir_suffix', gir_suffix)
+tests_conf.set('glib_datadir', glib_datadir)
+tests_conf.set('glib_girdir', glib_girdir)
+
+configure_file(
+  configuration: tests_conf,
+  input: 'glibconfig.py.in',
+  output: 'glibconfig.py',
+  install: installed_tests_enabled,
+  install_dir: installed_tests_execdir,
+  install_tag: 'tests',
+)
diff --git a/tests/meson.build b/tests/meson.build
index e5518c64d..42162f4c3 100644
--- a/tests/meson.build
+++ b/tests/meson.build
@@ -34,18 +34,4 @@ test(
   protocol : 'tap',
 )
 
-# TAP test runner for Python tests
-if installed_tests_enabled
-  lib_path = fs.relative_to(
-      meson.project_source_root() / python_test_libraries_path,
-      meson.current_source_dir())
-
-  install_data(
-    files(
-      lib_path / 'taptestrunner.py',
-      lib_path / 'testprogramrunner.py',
-    ),
-    install_dir: installed_tests_execdir,
-    install_tag: 'tests',
-  )
-endif
+subdir('lib')