His son is expected to succeed to the throne
President Michael D Higgins is leading tributes to the world's longest-reigning monarch, King Bhumibol Adulyadej, who has died aged 88.
Thailand has started a year of mourning after his death.
The palace there did not give a reason for his death, but said in a statement: "Although the team of doctors treated him to the best of their ability, his condition deteriorated.
"His Majesty has passed away at Siriraj Hospital (in Bangkok) peacefully."
Bhumibol ascended the throne in 1946 as an 18-year-old, after the death of his brother King Ananda Mahidol from a gunshot wound in a palace bedroom.
The circumstances of that death remain unexplained.
He was crowned in 1950, a week after he married Sirikit, and together they visited nearly 30 countries during his reign.
In a statement, President Michael D Higgins said: "On behalf of the people of Ireland I wish to convey my condolences to the people of Thailand on the death of King Bhumibol.
"With his passing, the world has lost both its longest reigning head of state and a statesman deeply committed to peace and peaceful co-existence.
"I wish to express my deepest sympathies to his wife, Queen Sirikit, and his children, in particular to his daughter Maha Chakri Sirindhorn whose visit to Áras an Uachtaráin I recall with fondness, as well as to his wider family and to the people of Thailand."
During a 70-year reign, the US-born king became his nation's one constant, as a long line of governments rose and fell.
He was remembered as a gentle leader, a frail-looking and softly-spoken man who used his influence to unite the nation and rally troops through the Cold War as Thailand's neighbours fell to communism.
But he was also down to earth - hiking to remote rice paddies and poor villages to examine the state of the country beyond the palace steps and to resolve various disputes.
King Bhumibol became the world's richest monarch during his heyday and lived the life of a modern king, racing yachts and wearing ornate clothing.
But he spent most of his final years in hospital, rarely seen in public and remaining silent on the upheaval and protests that have shaken Thailand in recent years.
The king's son and heir apparent, the 64-year-old Crown Prince Maha Vajiralongkorn, is expected to succeed to the throne.
Many commentators say he does not have the influence of his father, however.
Thailand's prime minister Prayut Chan-o-Cha said: "The government will proceed with the succession.
"The government will inform the National Legislative Assembly that His Majesty the King appointed his heir on 28 December, 1972."
This is the date that Vajiralongkorn was made crown prince and it is expected that he could be made king later this afternoon.
He also urged Thais to love one another and to preserve "the father's land".
The king's death will be the first major test for the generals who seized power in 2014, promising to restore stability after a decade of political chaos.
King Bhumibol's wife Queen Sirikit, has also been ill in recent years and rarely seen.
The death of King Bhumibol Adulyadej means Britain's Queen Elizabeth II, crowned in 1952, becomes the world's longest reigning monarch.