Rinse meat and pat dry. Cut 1/2-inch-wide slits all over roast. Cut garlic into 1/2-inch slices and tuck into meat slits. Rub meat with herbs. Place in 1-gallon heavy plastic food bag, add beer, and seal bag. Set in a bowl and chill at least 1 hour, or up to 4 hours; turn over occasionally.
Prepare barbecue. If using charcoal briquets, mound and ignite 60 briquets on the firegrate of a barbecue with a lid. When briquets are dotted with gray ash, in 15 to 20 minutes, push equal amounts to opposite sides of firegrate. Add 5 more briquets now, and every 30 minutes of cooking, to each mound of coals.
If using a gas barbecue, turn heat to high and close lid for 10 minutes. Adjust burners for indirect cooking (no heat down center) and keep on high.
Set a drip pan on fire grate between coals or burners. Set barbecue grill in place.
Lift meat from marinade and lay on grill, not over heat. Cover barbecue; open vents for charcoal. During the first 30 minutes, baste meat with marinade occasionally. Cook beef until a thermometer inserted in the center of the thickest part reaches 135° for medium-rare, 35 to 40 minutes. Discard remaining marinade.
Transfer beef to platter or board, keep warm, and let rest about 10 minutes. Cut meat into thin slices; add salt and pepper to taste.