Argthtjtr

You address the 90 non-consecutive day stay out of 180 days in the Schengen. In this explanation the 180 day count begins on the initial day of entry. You then have 90 (89)days within the next 180 days to spend in the Schengen. Can the next 180 period begin on day 181 without spending 90 days outside the Schengen? Can you tell me if the same rules apply to Ireland, where 90 - 189 day runs also apply? Or direct me to a source for this answer?

I believe I understand the Schengen 90 - 180 rules. My question is, are the rules the same for Ireland, since they are not a Schengen country? If the rules differ, how do the rules differ! I am. U.S. citizen and would be in Ireland using U.S. Passport.

Under the Schengen rules, you don't get to pick the 180 day period. You need to structure your visit(s) so that no 180-day period includes more than 90 days in the Schengen zone. For instance: Suppose you went to France for 60 days, went home for 80 days, then went to Germany for 40 days. In that situation you would have violated the limit, because the 180-day period which ended when you left Germany included 100 total days in the Schengen zone. But if you'd gone home for 95 days instead of just 40, you wouldn't be in violation, because no 180-day period would include more than 90 days of stay.

Basically: Every day you're in the Schengen zone, you need to be able to say "during the past 180 days, I've been here less than 90 days total". Where you were 183 days ago is not relevant.

Your question indicates that you do not quite understand the Schengen rule correctly, but the other answer may have changed that, and the focus of your question is the rules in Ireland, so I will address those rules.

The Irish Department of Justice and Equality has a page on single- and multiple-entry visas. It says

A multi entry visa permits you to travel to Ireland on a number of occasions during the dates shown on you visa, for short trips only. It is not permitted to use this category of visa to bypass immigration rules governing residency in the State i.e. it is not possible to remain in Ireland for a period of 90 days and then seek to re-enter the country for a further period of up to 90 days. Any abuse of this may result in you being refused entry at the port of entry or future visa applications being refused.

This implies that you can be admitted for up to 90 days on each visit, but that the officer at the border has discretion to admit you for a shorter period, or to refuse entry altogether, if you seem to be using the visa "to bypass immigration rules governing residency in the State."

Unlike the strict Schengen rule, the Irish rule allows officers to admit someone for a short trip, even if the person has recently spent 90 days in the country, as long as the officer is convinced that the person will leave as promised and that the trip has the character of a temporary visit. On the other hand, it means that Irish officers may need to scrutinize travel plans more closely than Schengen officers would, if the recent travel history includes a good deal of time spent in their jurisdiction.

If you would like to plan to spend significantly more than 90 days in Ireland in any 180-day period, however, you should proceed with caution, because an adverse decision by a border officer could ruin your plans and cost you a good deal of money in ticket change fees.