The pop star will play the Meath venue on Saturday, June 10, as part of his Love On Tour show.
It will be the first major concert at the Co Meath venue since the onset of the Covid-19 pandemic.
Speaking at a briefing today ahead of the gig, event controller Eamonn Fox said a “comprehensive and robust plan” was put in place to secure private coach services for the event.
Mr Fox said that approximately 30,000 concertgoers, out of a total of 80,000, will be travelling via public transport.
“On the buses, they may sell out and probably will in certain areas. There’s nothing we can do in that sense. We put on as many services as we can,” he said.
“We are bringing a city to a village.”
Bus Éireann Expressway routes to Slane Castle from George’s Quay, Dublin, Cork Bus Station, Parnell Place, Limerick Bus Station and Galway Bus Station are all sold out, Independent.ie has confirmed.
There is still availability from St Patrick’s School in Navan and Drogheda Bus Station.
Marathon Coaches are also sold out and have no additional capacity from Custom House Quay or Northwall Quay.
Irish Concert Travel has closed its booking service and is not accepting cancellations or operating a waiting list for transport to the event.
Alex Mountcharles at Harry Styles briefing in Slane Castle
On its website, it said no extra seats will be added and it is only dealing with queries from people who have purchased bus tickets already.
The company is providing transport to the concert from Donegal Town, Ballyshannon, Cavan, Sligo, Carrick on Shannon, Longford, Mullingar, Ballina, Foxford, Swinford, French Park, Strokestown, Castlebar, Claremorris, Ballyhaunis, Castlerea, Roscommon, Galway City, Galway Plaza, Ballinasloe, Athlone and Dublin.
Buses To Concerts has also stated on its website that all travel to Slane Castle is sold out.
It is operating routes from Drogheda, Dundalk, Belfast, Newry, Banbridge and Lisburn.
Halpenny Transport in Co Louth, which is providing travel from Dundalk, is also fully booked.
Travel Master is operating a nationwide service and has added extra seats, but many are already sold out.
Other sold-out services include Michael Friel Coaches and Boyce Coach Travel in Co Donegal.
Having already scrambled to secure tickets to the gig itself, fans are now scrambling to secure bus tickets.
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For those driving to the event, all car parks are independently operated and must be booked in advance. All public car and bus parks are a 20- to 30-minute walk from the site.
There will be four official car parks – green, blue, red and pink. Car parks are assigned when booking based on garda traffic management plans.
Fans have been advised to prepare well in advance to ensure an enjoyable experience.
Gates open at 2pm, and the first act will take to the stage at 2.30pm.
Organisers will not allow attendees to queue before the designated start time and no re-admission is allowed.
Under-16s must be accompanied by a parent or guardian over 25 and unaccompanied minors will be refused entry.