More than 50,000 visitors are expected in Dublin this weekend for the annual St Patrick's Festival.
The national holiday celebrations centre round tomorrow's St Patrick's Day parade through the streets of the capital.
President McAleese
Hundreds of thousands are expected to attend the spectacle although the finish clashes with Ireland's bid to win the Six Nations Rugby Tournament for the first time in 22 years, when they take on Italy in Rome.
The weather on St Patrick's day will be mostly cloudy and quite windy, according to Met Éireann .
Many areas will be dry, it said, but rain or drizzle in the north and northwest will spread southeastwards during the late afternoon and evening with highest temperatures between 9 and 12 degrees.
The Road Safety Authority is urging motorists to exercise extreme caution over the holiday period due to the extra weekend traffic.
A RSA spokesman said: "Weekends are high risk periods on our roads but over bank holiday weekends those risks increase significantly."
"This bank holiday the RSA is supporting the enforcement activity of the gardaí who will be conducting extensive mandatory alcohol testing checkpoints over the weekend. The message for drink drivers is that the chances of getting caught and losing your licence are high," he said.
Iarnród Éireann has laid on extra InterCity, DART and commuter services for Saturday's parade and for Sunday evening's Skyfest which begins with a concert by the Waterboys at 7pm in Dublin's Docklands.
The Dublin Airport Authority said it expected some 330,000 passengers through Dublin Airport over the course of the weekend.
Over 2,100 flights will arrive and depart during the weekend including more than 200 charter flights, it said.
The DAA urged passengers to familiarise themselves with the EU Security Regulations before setting out.
President Mary McAleese said the St Patrick's Day parades linked Ireland's global family and its many friends in a huge celebration "of the green" all over the world.
"The Irish love of music, dance, fun and friendship will be showcased in every continent thanks to the work of countless committees whose passion for Ireland and her unique heritage has seen the St Patrick's Day pageant grow into a truly global phenomenon.
"I thank them all for this outstanding network which connects Ireland to her children and her friends in such a joyful way," she said.
In Galway the St Patrick's Day parade returns to Eyre Square for the first time since the refurbishment. The theme of this year's spectacle Paddywood featuring a series of sketches based on Hollywood in an Irish setting.
Cork's three-day festival will begin tomorrow with the parade and several live concerts. Festival organisers in Limerick are planning the largest parade yet under the theme of Irish Myths and Legends.