SHANNON, Ireland — An ocean away from the United States, travelers flying out of the international airport here on the west coast of Ireland are confronting one of the newest lines of defense in the war on terrorism: the U.S. border.

In a section of this airport carved out for the Department of Homeland Security, passengers are screened for explosives and cleared to enter the United States by U.S. Customs and Border Protection officers before boarding. When they land, the passengers walk straight off the plane into the terminal without going through border checks.

At other foreign airports, including those in Madrid, Panama City and Tokyo, U.S. officers advise the local authorities. U.S. programs in other cities expedite travel for passengers regarded as low risk.

The programs reflect the Obama administration’s ambitious effort to tighten security in the face of repeated attempts by al-Qaida and other terrorists to blow up planes headed to the United States from foreign airports.

The thinking is simple: By placing officers in foreign countries and effectively pushing the U.S. border thousands of miles beyond the country’s shores, Americans have more control over screening and security. And it is far better to sort out who is on a flight before it takes off than after a catastrophe occurs.

“It’s a really big deal — it would be like us saying you can have foreign law enforcement operating in a U.S. facility with all the privileges given to law enforcement, but we are going to do it on your territory and on our rules,” the secretary of the Department of Homeland Security, Janet Napolitano, said on a flight back to the U.S. from the Middle East, where she negotiated with leaders in Israel and Jordan about joint airport security programs.

“So you flip it around, and you realize it is a big deal for a country to agree to that. It is also an expensive proposition.”

Airports in 14 countries are participating in the programs, which have been expanded over the past several years and have required substantial concessions from foreign leaders. In many cases they have agreed to allow U.S. officers to be placed in the heart of their airports and to give them the authority to carry weapons, detain passengers and pull them off flights.

In December, the government of Abu Dhabi signed a letter of intent to build a terminal where U.S. officers will clear passengers to enter the United States, the most ambitious agreement the United States has struck so far with an Arab country. On her recent trip to Jordan, Napolitano began negotiations with the ruling family there about similar efforts.

Peter King, R-N.Y., chairman of the House Committee on Homeland Security, endorsed the security efforts and said he hoped the department would expand them.

“A lot of these attempts are coming from the Middle East,” he said, referring to terrorism plots, “and that drives home that we have an immediate problem and that we need to push for these programs there as hard as we can.”

The Obama administration sped up expansion of the programs, which cost about $115 million a year, after an al-Qaida operative tried to detonate explosives hidden in his underwear on a flight from Amsterdam to Detroit on Christmas Day in 2009. The security at foreign airports drew fresh public attention last month after new reports that intelligence agencies thwarted another plot by al-Qaida to detonate an underwear bomb on a U.S.-bound airliner. After that news emerged, Napolitano said the new measures being put in place in foreign airports for flights to the United States would have stopped a terrorist from boarding a plane with such a bomb.

But critics of the department on Capitol Hill — particularly two Republican committee chairmen in the House, Darrell Issa of California and John Mica of Florida — questioned her claims and said that security in foreign airports is not robust enough.

Napolitano and other Obama administration officials praise the programs as essential to help protect the 80 million passengers a year who fly to the United States from 300 foreign airports, and a boon for travelers, who save time after landing.

Still, as with many other counterterrorism measures, it is hard to gauge the programs’ success or their impact on al-Qaida and other terrorists. They have not foiled any major plots so far, and it is hard to imagine terrorists unaware of which airports had a robust U.S. security presence and which were more vulnerable.

Homeland Security officials acknowledge that the United States cannot control security in every airport. The focus, they said, was on expanding a U.S. presence at airports with a significant number of U.S.-bound flights.

There are still hundreds of airports where U.S. officials have less say. According to a Government Accountability Office report in October, the Transportation Security Administration “has identified serious or egregious noncompliance issues at a number of other foreign airports.”

Department officials, however, said they were confident in the security at foreign airports because even those that do not have pre-clearance or U.S. advisers are subject to audits by the TSA.

“There was an airport that a U.S. flight carrier wanted to use as a last point of departure two years ago, and we denied it because we couldn’t ensure that planes couldn’t be hit by a shoulder-fired missile near the airport,” Napolitano said.