Have you ever chaperoned a field trip? One where all the kids are drunk? And you’re afraid one of them might hijack the school bus? If so, you have a general idea of what it’s like to ride the Erin Express, the bar promotion that is unleashed on Philadelphia each spring like a recurring nightmare, turning the city into one big, Kelly-green-clad, beer-soaked Hieronymus Bosch painting.

This understanding of how very far I am from keeping up dawns on me around 11 a.m., as I breach the armada of port-a-potties and idling school buses and enter Cavanaugh’s University City to find a somehow-already-drunk crush of youthful humanity. Within an hour, I will have reunited several stumbling, emerald-attired individuals with various photo IDs and transit passes that have fluttered to the floor, unnoticed.

“It doesn’t even work,” one says, pocketing a credit card that had slipped from his pocket. “I keep it because it’s metal. I used it to chop up some Ritalin earlier.”

This sprawling event — the grandfather of Philadelphia bar crawls — is celebrating roughly its 40th anniversary this year. On St. Patrick’s Day 1979, it ran on a commandeered Fairmount Park trolley. Today, the celebration has spilled over to two weekends, half a dozen school buses, more than a dozen bars and the echoing cavern of the 23rd Street Armory. It is a mess, but an exceedingly well-run mess; I never have to wait long to order a drink, use a bathroom, or hop a bus. (An exception is the event website, which is filled with conflicting and outdated times, dates, and locations.)

The benefits of barhopping are not entirely clear to me. Each new venue just brings more 22-year-olds dancing to “Jump Around” in shamrock-green tutus, sequined suspenders, or gag T-shirts playing on themes of Ireland (“Who’s your Paddy?”), binge drinking (“Take a pitcher, it’ll last longer”) or some combination of the two (“Irish Drinking Team”). Besides, this is not how to see a bar at its best. Consider Blarney Stone: Normally a charmingly ramshackle, student-centric Irish pub in University City, this weekend it’s huddled behind a defensive line of port-a-potties, with a prophylactic tarp spread over the pool table.

Blarney Stone during Erin Express is behind a defensive line of port-a-potties.
Samantha Melamed / Staff
Blarney Stone during Erin Express is behind a defensive line of port-a-potties.

On the other hand, there is free transportation. So I get in line for the school bus. Most of what happens while aboard is unprintable, but highlights include: people passing around a lemon squeeze bottle (“It’s got molly in it,” a guy says, maybe joking; I’m too old to be sure); a man describing himself as from “outside West Philly,” and then, pressed for clarification, “Lower Merion. You ever heard of Kobe Bryant?” Everyone seems to agree that living back with their parents is the worst, Alpha Something Something is the best fraternity, and Center City Sips is dope as f-.

Because Cavanaugh’s runs the event, along with Smokey Joe’s, I decide to make it a Tour de Cavanaugh’s. (The management didn’t respond to my interview request.)

I find that, as I head east, each Cav’s location is successively more pleasant and less crowded with students.

The University City location, a campus mainstay for 80 years, reminds me of the apartments I rented in college, the landlord decorating according to the school of "don’t make it too nice, they’ll just trash the place.” There’s the Rittenhouse location, which is a standard Center City sports bar (this weekend, with all the furniture removed, glasses swapped out for plastic cups, and a DJ playing, you guessed it, “Jump Around”).

My last stop is Headhouse Square, in a building that’s long housed classic English pubs (Dickens Inn, then Dark Horse) in a warren of quaint, inviting rooms with vintage decor and college basketball on flatscreens. It’s big enough to accommodate a mix of greenies and civilians — with dancing in one room, giant Jenga in another. Off in one area, a group of guys surrounded by laptops, beers, and half-eaten plates of wings are conducting a fantasy baseball draft. (“Every guy who comes in here is like, ‘Cool!’ ” they explain, looking up from their spreadsheets. “Every woman is so unimpressed.”)

After each bar, I emerge blinking into a sunny Saturday afternoon with the disoriented sensation of leaving a theater after a matinee, wondering what it is I’m doing with my life.

I’ve been collecting theories as to why St. Patrick’s Day has become such an event: An excuse to day-drink. Fetishization of Irish heritage. Alt-Halloween. A variation on a pagan rite of spring. I’m still not sure, but five hours of drinking and three Guinnesses later, I think I’m OK not knowing.

St. Patrick's Day, 1983: The Erin Express ran on a Fairmount Park trolley back then.
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St. Patrick's Day, 1983: The Erin Express ran on a Fairmount Park trolley back then.

Cavanaugh’s

119 S. 39th St., 215-386-4889; 1823 Sansom St., 215-665-9500; 421 S. Second St., 215-928-9307

When to go: Erin Express buses run roughly from noon to 6 p.m. on Saturday, March 16.

Bring: A friend you can trust to make sure you get home safely.

Order: What else? A pint of Guinness ($6.60) and a shot of Jameson (also $6.60).

Bathroom situation: In addition to acres of portable toilets (and, unfortunately, untold alleyways), the bars on this crawl show serious commitment to bathroom management. Cavanaugh’s University City has posted drill-sergeant-like bathroom attendants, both directing traffic and cleaning, to keep order.

Sounds like: A noisy 100 decibels, with current and past dance hits and DJs mostly skipping straight to the hook. This just might be the first time “Despacito and “Born to Run” have been played back to back.

Erin Express crowds outside Cavanaugh's.
Samantha Melamed / Staff
Erin Express crowds outside Cavanaugh's.