About 275,000 rugby fans are estimated to be in Cardiff for Saturday's Six Nations decider where Wales aim to beat Ireland to claim the Grand Slam.
It would be a clean sweep of victories in the tournament but Ireland could still finish top with a win.
Extra flights have been laid on from Dublin, however, the bad weather has scuppered plans for a giant screen.
One city centre landlord expects to sell 35,000 pints at his pub for the match which starts at 14:45 GMT.
"Our Celtic cousins enjoy a party as much as us," said Nick Newman, who runs the Philharmonic and is also chairman of Cardiff Licensees Forum.
Match day is here #WALvIRE 🏉 wherever you’re watching the matches, please enjoy. Please also remember if you’re drunk, you will not be allowed into bars and clubs. If someone is refused entry in your group, stick with them and don’t leave them on their own #DrinkLessEnjoyMore pic.twitter.com/fAY49gDbz2— South Wales Police (@swpolice) March 16, 2019
With so many fans in the Welsh capital, it is no wonder bars have ordered extra stocks of beer and, with St Patrick's Day on Sunday, particularly Guinness.
On what is set to be a "bumper day", one manager has even positioning vans around the city with extra stock to make sure no-one runs dry.
The Philharmonic is predicted to sell 35,000 pints on Saturday while JD Wetherspoon expects to take more than £200,000 across its three pubs in the city centre.
Management at the Gatekeeper - opposite the Principality Stadium - estimate bar staff will pour more than 100 pints per minute for the whole day.
Walkabout - the biggest sports bar in town with a capacity of 1,400 - has even bussed in 180 kegs of beer.
Mr Newman said: "The Wales-Ireland game is always special. This will be five times bigger than a normal Saturday.
"We've been building to this obviously for some time. As soon as we beat England we kind of realised there was going to be an awful lot on this weekend.
"The kick off time really suits us tomorrow as people will be out early having some food and drink before the game and then they'll have the whole night to go.
"If I think back to 2005, I was running a big pub on St Mary Street and we actually had to have an emergency delivery at six in the morning."
Some 50 additional flights have brought thousands of Ireland fans to south Wales on St Patrick's Day weekend meaning many hotels and guest houses in the capital are full.
However, Cardiff council confirmed on Friday that they will not be facilitating a big screen this year, saying the weather forecast "creates too much risk" for such an open-air event.
A spokesperson added: "Whatever the weather, the capital will afford every visitor a fantastic welcome on a day which will be a huge boost to local businesses."
The Principality Stadium roof will be open for the match where Wales are aiming for a third Grand Slam under Warren Gatland who takes charge for his last Six Nations game as coach.
The bad weather has also led to warnings from South Wales Fire and Rescue Service for fans to stay away from water.
Up to 275,000 people are expected in Cardiff on Saturday, when Wales take on Ireland with hopes of a Grand Slam victory.— Cardiff Council (@cardiffcouncil) March 12, 2019
There will be a full city centre road closure between 11.15am and 5.45pm, with kick off taking place at 2.45pm. Read more here: https://t.co/2M7BAsvvKq pic.twitter.com/YMU0QeLwey
Information on local buses can be found on Cardiff council's website.
Trains in the Cardiff area are expected to be very busy all day, with people travelling into the city advised to travel as early as possible.
Transport for Wales has confirmed Cardiff Queen Street Station will close at 16:15 and there will be a post-match queue system in place at Cardiff Central Station.
A full city centre road closure is in place until 17:45, including:
- Kingsway from its junction with North Road to its junction with Duke Street
- Cowbridge Road East from its junction with Cathedral Road to its junction with Westgate Street
- Tudor Street from its junction with Clare Road to its junction with Wood Street (access for residents and traders will be permitted via Fitzhammon Embankment)
- Plantagenet Street and Beauchamp Street from their junctions with Despenser Place to their junctions with Tudor Street (access for residents and traders will be permitted)
- Saunders Road from its junction with St Mary Street
- Customhouse Street in its entirety (access to private car parks will be permitted)
- Penarth Road from its junction with Saunders Road to the entrance leading to the rear of the Central Train Station
- The following roads will be closed in their entirety: Duke Street, Castle Street, High Street, St Mary Street, Caroline Street, Wood Street, Central Square, Westgate Street, Quay Street, Guildhall Place, Golate y Gwter, Park Street, Havelock Street and Scott Road