With more than 60 million sold, it generates almost as much buzz as the popular McRib sandwich.

The Shamrock Shake--the bright green sweet treat that signals that St. Patrick's Day is right around the corner-- is another long-time McDonald’s cult favorite and for more than four decades is still going strong. But have you ever wondered what’s really in it?

The Shamrock Shake then and now. (Courtesy McDonald's)

“It’s like little green are Leprechauns crawling down my throat,” one fan we stopped in the streets of New York told us. “With some little grounded up four-leaf clovers.”

“I taste some nutmeg,” said another. “Maybe a little grenadine.”

Sorry folks, there are no leprechauns or nutmeg on the list of ingredients.

But we did get McDonald’s archivist Mike Bullington to give us the inside scoop about how it got its start and how it got its staying power.

Who invented the Shamrock Shake?

Philadelphia Eagles Players present jersey to Ray Kroc at the first Ronald McDonald House, 1974. A week-long Shamrock Shake promotion funded the purchase of the four-story, seven-bedroom house which allowed families to stay close to their kids being treated in the hospital nearby. (Courtesy McDonald's)

The Shamrock Shake was invented in 1967 by Hal Rosen, a Connecticut franchise owner and operator, who concocted the drink to commemorate St. Patrick’s Day. Debuted nationally in 1970, today it usually appears at the end of February and is sold through the Irish holiday. But did you know that the seasonal shake was almost an instant success and helped generate enough funds to help with the opening of the very first Ronald McDonald House in Philadelphia in 1974?

What is the real flavor of the Shamrock Shake?

In the beginning, “individual McDonald’s franchisee determined if the shake was green mint flavored or green vanilla flavored,” says Bullington. That’s right, it wasn’t always minty. By 1980, however, McDonald’s corporate decided the shake should be mint flavored and the tradition stuck.

Today the Shamrock Shake has four main ingredients: vanilla soft serve ice cream McDonald’s uses in all shakes, Shamrock Shake Syrup (which contains dyes and artificial flavoring), whipped cream and a maraschino cherry. It may be delicious but it probably shouldn’t be consumed every day. A medium is 660 calories—which is more than a Big Mac.

Who drinks the most Shamrock Shakes?

McDonald’s wouldn’t release its official yearly figures but says the shake is still most popular in Philadelphia, followed closely by Michigan.

Is it available everywhere?

Franchise owners can decide whether they want to carry the shake when it's in season but if you're in the U.S. there's a good chance your local McDonald's is serving it. But if you’re a devoted Shamrock Shake fan and you find yourself out of the country on St. Patrick’s Day, good luck. Internationally, it's only available in Ireland and Canada.

What happens to the green syrup when it’s not March?

Once it’s gone, it’s really gone. The syrup for the Shamrock Shake is ordered just for the spring season and is sold until it runs out.