The letter was contained in files allegedly found at bin Laden's Pakistan hideout after he was killed by U.S. special forces in Abbottabad, Pakistan, last May.

The Combating Terrorism Centre, a privately funded research base at the U.S. Military Academy at West Point, posted a number of declassified documents belonging to bin Laden on its web site May 3. They were taken in the raid on his house.

The letter from Gadahn particularly highlighted the reason for approaching the Irish, noting Ireland was not a participant in "Bush's Crusade wars."

It noted "the increasing anger in Ireland towards the Catholic Church after exposing a number of sex scandals and others" and speaks of the hunger of youths because of the economic downturn in Ireland.

Gadahn wrote that Irish people, "who were the most religious of atheist Europe," were moving toward secularism.

"Why do not we face them with Islam?" he asked.

He said he considered preparing a similar message to Catholics living in Arab regions, calling them to Islam: "Catholics were historically the prominent enemies of the Jews, amongst the other Christians. They were also the original enemies to the evangelist Protestant(s) who were the vanguard of the Crusades.

"Their public in general, these days, is more sympathetic and understanding of the Muslims, than other Protestant and Orthodox Christians," he said.

However, he said he put off his message following backlash from an attack on a Catholic church in Iraq. Fifty-eight people died in an attack on the Syrian Catholic Church in Baghdad Oct. 31, 2010, after military officials tried to end a terrorist siege of the church.