Terror is worse than war, the Chief Rabbi wrote. "In war, people know the rules and risk; in terror there are no rules and everyday life becomes a risk."
At the synagogue in Terenure, Dublin, yesterday, coming together to pray was played out to a new set of rules. Gardai stood at the door and gateway and men in dark suits conducted discreet security checks on every unfamiliar visitor.
The Jewish community in Ireland is without its leader at present and the horror that brought them to the synagogue may keep him from them a while longer.
Newly-appointed Chief Rabbi Yaakov Pearlman was due to leave New York for Ireland tomorrow, but his congregation is unsure when it will be able to welcome him.
In his absence, Rabbi Zalman Lent and Cantor Alwyn Schulman led the congregation in prayers for the victims of the attacks in the US.
"At this time of such enormous tragedy and suffering, words are pitifully inadequate to describe the hideous and unspeakable massacre of innocent people which took place," said Rabbi Lent.
"To comprehend the enormity of the tragedy is surely beyond us. All we can register is shock and disbelief. The pain is yet to come."
Rabbi Lent read a special message from Chief Rabbi Jonathan Sacks in Britain, who described the atrocity as "the day when the world witnessed the naked face of suicidal hatred, malign in its target, global in its reach, devastating in its effect on human life".
With a note of hope, he said violence would never win. "The human spirit is stronger than any attempt to crush, intimidate or terrorise it."
Visitors from the US among the congregation were overwhelmed by the number of people who had thrust scraps of papers with hastily-written names and phone numbers into their hands, insisting they come to dinner if their flight home today is cancelled.
Mr Jason Zellner and his wife, Marcia, were on holiday with two other Miami couples when they heard Tuesday's terrible news.
"We feel like we are displaced. My first instinct was to get an American flag," said Mr Zellner, revealing the Stars and Stripes badge pinned to his shirt. "I was a little bit concerned wearing it. I was proud to be American yet I felt like I was vulnerable."
The Zellners and their friends were fortunate not to have lost anyone in the attacks but they still felt the tragedy deeply.
"I've called home and the streets are empty, there are bomb scares and the federal building was evacuated."
Ms Zellner said the response of the Irish had helped her deal with the shock and dismay. "People deliberately sought us out to say how sorry they were."