Next week's Irish Open at Royal Portrush will be the first sell-out the European Tour has ever had for a regular event.
No tickets are left for the final two rounds and none will be available for the opening two days after 5pm on Friday.
It is the first time the tournament has been held in Northern Ireland since 1953 and local heroes Rory McIlroy, Darren Clarke and Graeme McDowell are all taking part along with USPGA champion Keegan Bradley and three-major winner Padraig Harrington.
"We are preparing for a crowd around the 100,000 mark for the week," championship director Antonia Beggs said.
Northern Ireland Tourist Board chief executive Alan Clarke said: "This year's Irish Open is sure to be one of the biggest in the competition's history with advance ticket sales reaching unprecedented levels and record crowds expected.
"While in previous years it may have been possible for spectators to turn up on the day and purchase tickets on the day, we would strongly urge people not to try this on Saturday and Sunday as the event is completely sold out at the weekend and there is absolutely no chance of being admitted without a ticket."
In addition to normal scheduled services, extra trains will run from Belfast and Derry-Londonderry to Portrush and there are dedicated coaches from various locations.