The weather is expected to be mild, bright and mostly dry over the final bank holiday before Christmas, but motorists are being urged to drive safely over the next three days as thousands take to busy roads.
The National Safety Council has chosen this weekend to launch its Fly the Flag campaign because bank holidays tend to the periods when road fatalities increase significantly.
This weekend over 4 million flags, designed to be attached to car aerials and clothing, are available in 1,500 outlets throughout the Republic. The flags are there to remind motorists, particularly the most vulnerable group between 20 to 34 years of age, about road safety.
The number of people travelling is also likely to create large tailbacks, says AA Roadwatch, with Monday evening particularly bad.
Met Eireann says temperatures during the day will range from 10 C to 16 C, with generally light winds from the south-east and by Monday from the north-west. There is a chance of fog, particularly on tonight and tomorrow morning.
Two events in Cork - the funeral of Mr Jack Lynch and the Jazz Festival - are expected to cause traffic congestion in the city, and could be particularly bad this morning. "One of the busiest routes will be the N8 Dublin/Cork road," Mr Trevor Keegan of AA Roadwatch says.
The participation of 6,500 runners and walkers in the Dublin City marathon will mean many roads will be closed on Monday morning and afternoon.
AA Roadwatch advises motorists travelling on Monday to use the M50 or the East Link toll bridge if they need to go across the city.
The Galway races will bring an influx into the west tomorrow and Monday, while Co Donegal will have extra traffic attending the Bundoran Music Festival, which takes place today and tomorrow.
Bus Eireann expects to carry 100,000 passengers on Express Way and provincial bus services. Services on Monday will operate as on Sundays with a few alterations and extra services to cater for people returning to the main centres that evening. Iarnrod Eireann advises passengers to reserve their seats on several rail services this weekend due to the excessive demand.
Stena Line expects to carry 40,000 passengers and 8,000 cars on Dun Laoghaire/Holyhead, Dublin Port/Holyhead and Rosslare/Fishguard routes over the holiday period.
Aer Rianta expects more than 155,000 passengers to take 1,300 journeys from Dublin Airport this weekend. A spokesman advised travellers to allow sufficient time to reach the airport in time to check in. Aer Rianta's three airports - Dublin, Shannon and Cork - expect to handle 250,000 passengers.