Place corned beef fat side up in crockpot.
Sprinkle spice packet and peppercorns over meat.
Layer sliced onions on top.
Pour Guinness slowly into crockpot.
Add cold water to just cover brisket.
Cover and cook on low for 8-10 hours until fork tender.
Check liquid level at 6 hours and add water if needed.
Remove brisket when fork slides in easily and wrap tightly in foil.
Rest for 30 minutes.
Unwrap and scrape off peppercorns and onions.
Remove and discard fat cap.
Slice against the grain into thin slices.
Spoon cooking liquid over sliced meat before serving.