Newton first realized that every action requires an equal and opposite reaction when, upon serving a burger to a friend, the friend promptly invented beer. Enjoy a tasty side of science with today's Groupon: for $15, you get $30 worth of pub fare and drinks at P.J. O'Brien's Irish Pub and Restaurant.

P.J. O'Brien's surrounds customers in the charm and atmosphere of an authentic Irish pub and sates stomachs with a team of four pub menus and a roster of more than 20 Irish whiskies. Chefs marry braised beef and Guinness in a stew stuffed with potatoes and parsnips ($15) without fussing with the pyrotechnics of traditional weddings. PEI mussels and vegetables frolic in white wine and tomato sauce ($11 for 1 lb.), and grilled Atlantic salmon takes a dip in a lemon-caper butter sauce ($19). Draft brews such as Alexander Keith's IPA and Guinness depart from the bar's spouts in a pour as fluid as James Joyce's stream of consciousness. At Saturday-morning brunch, diners mine a full Irish breakfast for deposits of eggs, bacon rashers, homemade sausages, black pudding, baked beans, and roast potatoes ($9.95).

Plasma TVs bathe guests in the adrenalin-charged light of live Maple Leafs and Raptors games. On Friday and Saturday nights, patrons waltz in 3/5 time to live traditional Celtic and contemporary music performed on the restaurant's stage. Guests dine in the main dining area and two private lounges, surrounded by Irish-style wood panelling and copper-topped bars. Six nights a week, P.J. O'Brien's entertains revellers until 1 or 2 a.m.