I think it is for the following reasons, most of them pure commonsense:

a) Political and cultural ties between the two countries.

b) Safety and convenience; Most passengers coming from Ireland and probably all from Shannon are likely to be extremely low-risk (and at least half are US citizens) as regards terrorism or a desire to overthrow the US Government – particulary in the case of Shannon which now has full preclearance and no connecting flights from “dodgy” countries. Someone not fittng the terrorist low risk profile would stand out like a sore thumb here. It makes a quicker process for passengers coming from Ireland and it makes for a better experience for passengers arriving in the USA from other destinations – as Homeland security has slightly less people to process and therefore can do so more thoroughly and fairly as there is a little less time pressure.

c) Shannon has nearly 3 million US soldiers transitting it every year, ahving US pre-clearance makes even more sense for this reason.

I had been a lyoal NW customer for years. Unfortunately it appears Delta really doesn’t want or care about DTW business. If they want to “win” NY, fine, but at the cost of how much other business? Go ahead – “win” NY, and I will fly US Air

My impression is that the situation in the other small upper midwest towns with Northwest non-stops to DCA (Des Moines and Grand Rapids) is similar.

Judging by the typically-high fares and the fact that the flight has been mostly full most times I’ve flown it, the MSN-DCA flight that Northwest has been running for a few years must be successful.

So I just tried to book a Sept flight from LGA to CLT — but the always-overbooked 6:55pm is now GONE!!

It looks like you may have just picked a bad day. It’s not gone, but there are some quirky days in September that don’t have it operating. It’s now a 659p departure and it runs every day until 9/13. Then it doesn’t operate on Monday and Tuesday. There’s also a 735p Wednesday through Sunday, but Monday and Tuesday, for whatever reason, have neither of those.

Regarding Canadian preclearance: I thought the rationale was to allow flights from major Canadian airports to smaller airports in the U.S. which do not have immigration and customs facilities. But are there any scheduled flights left on such routes?

You’d be surprised at the number of flights that go to smaller airports. Air Canada will fly you from Toronto to places like Allentown, Harrisburg, and Dayton. I believe they all have customs and immigration facilities, but it’s probably cheaper and easier to just have it staffed in Canada.

I understand why it makes sense to have preclearance in Canada, but I don’t understand why they have it in Ireland. Anyone?

It could be political. I don’t know about the preclearance, but I was told that adding Ireland to the visa waiver program was done after a promise that Bill Clinton gave on a presidential visit to Ireland, despite the fact that at the time Ireland did not meet the State Department’s criteria for joining the program.

Regarding Canadian preclearance: I thought the rationale was to allow flights from major Canadian airports to smaller airports in the U.S. which do not have immigration and customs facilities. But are there any scheduled flights left on such routes? There’s LGA and DCA, but these are by no means small airports. So what’s the benefit (to the U.S. government/taxpayer) of Canadian preclearance?