What is the best way to fly into Belfast from the US? (OK wiseguy - I know, on an airplane!!) Serious replies only!!

Your main options are via EWR (CO), or LHR (BD/EI). There are other low-cost options from the UK mainland, but you'll need to re-check (with no guaranteed connections), and may be subject to harsh baggage restrictions and/or fees.

As an alternative, if you have an easy direct flight to Dublin, there is a bus from Dublin airport direct to Belfast, taking 2h25. Note that, though, transiting via Ireland, it does get a little complex as far as immigration is concerned.

Why does it get complex for immigration? There are no border stops between the north and south of the island. The bus from Dublin airport into Belfast is great.

As moremilesplease has said there is no border stop going from the republic of Ireland to northern Ireland. The road from Dublin to the north is very good, motorway most of the way so via Dublin is a good option for you.

This is precisely why it makes immigration issues complex. If you are an EU national or someone with the right of abode/permanent residence in the UK (indefinite leave to remain), then you have no problems. If, however, your immigration status requires you to be admitted into the country (given leave to enter the UK), the problem is that there is no-one to actually give you that leave as there are no UK immigration officers either at DUB or at the notional border points between the South and Northern Ireland, so you could find yourself an unintentional illegal immigrant in the UK. Not sure whether there are any possibilities for regularisation currently or if you are required to enter via a UK immigration point in such a situation.

Irish (Rep of)Immigration will be cleared at Dublin airport and there is freedom of movement for anyone throughout the island of Ireland after that. There is no complication whatsoever.

Now, I'm not saying you're wrong, I'm just saying that this is very counter-intuitive.

Northern Island is part of UK sovereign territory and subject to British immigration controls (i.e., if you fly from outside of the British Isles to Belfast directly, you go through British immigration when entering). The ROI has a different set of immigration controls. Doesn't that preclude free movement throughout the entire island?

Ultimately, you can cross over the border from one sovereign nation's territory to another's, but that doesn't mean you're there legally.

Yep. I agree with this. I had a situation a few years back before my wife became a US citizen. She had a visa to go to Ireland and we transited via LHR. Well, you actually enter the UK (not just transit) when you go from LHR-DUB and we did not know that. It was not a huge deal as the immigration guy just gave my wife a temp visa for the UK.

In the scenario above, had my wife had a Irish visa and then entered N. Ireland via land....she would technically have been there illegally.

The immigration problem would be likely to arise for a person who needed a visa to enter the UK.

The UK Immigration Rules (Part 1, no. 15) state:

By "merely passed through the Republic of Ireland" is meant, I believe, the situation in which the traveller did not clear Irish immigration. (However, although this provision exists, the situation does not arise as airports are presently set up, since, as noted above, there is no routine immigration control between the Republic of Ireland and Northern Ireland and anyone transferring from an international flight on to a Belfast-bound flight would pass through immigration.)

There are, however, sometimes ad-hoc controls at the border, so if you do require leave to enter the UK (i.e. you are not a British citizen, a Commonwealth citizen with the right of abode, a citizen of an EU/EEA country or Switzerland, or a person with leave to remain in the UK), then it would be sensible to have details confirming your length of stay in the UK, etc, to hand. If you fly between the UK and Ireland, you are more likely to have to show your passport to an immigration officer travelling to Ireland than from Ireland (certainly for trips between Great Britain and the Republic).

If you require a visa to enter the UK (i.e. if you are a visa national or if you are coming for other than a short-term visit), you will need to have a UK visa (an Irish visa would not suffice) and it may need to be "activated" after arrival in the UK.

Assuming that you are not a visa national and don't need a visa to enter the UK for some other reason, then you ought to be fine, but the Irish authorities will (I think) give you permission to remain in Ireland (and therefore in the Common Travel Area) for 3 months, rather than the 6 months' leave to remain that would normally be granted at a UK port of entry. Obviously, if your stay is for less than 3 months that is not a problem.

Changes are being proposed to provisions for travel between the UK and Ireland: whether any changes would make it all more straightforward remains to be seen!

This is incorrect. Not all third country nationals have an automatic right to travel freely in the common travel area (see the legislation referenced by Christopher). An Irish immigration officer cannot give you leave to enter the UK and if your immigration status requires such a leave (typically: if you need a visa), you need to enter the UK via a UK port of entry rather than across land borders in the common travel area.

It has been a very long time since there were flights between Dublin and Belfast. There were flights between Cork and Belfast and between Dublin and Derry. Not sure whether either service still exists. As a matter of principle, however, what you say is right.

In the hundreds of times I have travelled between Dublin and Belfast in the last ten years, whether by train or by car, I have yet to be subject to cross-border immigration checks. The only period I was regularly stopped at the border was during the foot and mouth epidemic but the controls were just to check whether animal products were carried rather than for immigration purposes. What would have a greater likelihood of happening is being nipped by the NIPS for a motoring-related offence (eg speeding) or some other non-immigration related random control.

They might be billed as "security checks" or some other thing rather than formal immigration checks. They are also targeted at people deemed likely (presumably by their appearance) not to have the proper papers.

You are there legally, as the United Kingdom and Republic of Ireland operate a common travel zone policy. Clear immigration in Dublin, and you are free to travel anywhere on the island of Ireland. Should you want to fly or go by ferry to Britain, you will be subject to the same id verification as you would for a domestic flight in the US.

Except, as noted above, in some cases, which probably wouldn't affect the OP anyway.

99.99% plus of people are not affected by this.

There is freedom of movement on the entire island of Ireland and once you clear immigration in the Republic of Ireland you are free to travel to northern Ireland. There is no ad hoc checks by UK immigration unless they have a tip off about drug smugglers or similiar. Most if not all of this was in place prior to the Good Friday Agreement.