Mike: Here's the details on the Schengen entry/exit regulation you asked about:Excerpt from the text of Article 20, Section 1 of the Schengen regulation (my comments in brackets]: "Aliens not subject to a visa requirement [US and Canadian citizens, among others] may move freely within the territories of the Contracting Parties for a maximum period of 3 months during the 6 months following the date of first entry."
The 25 countries in the "Schengen zone" are: Italy, France, Germany, Austria, Belgium, Czech Republic, Denmark, Estonia, Finland, Greece, Hungary, Iceland, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovak Republic, Slovenia, Spain, Sweden, and (since Dec 2008) Switzerland.
According to the US embassy website (in Poland): A tourist may enter and leave the Schengen area multiple times, except that the total number of days spent within any of these countries cannot exceed 90 days during the 6 months following the date of first entry.
It's not 90 days in each country; and the 90-day "clock" is not reset by making a quick trip to the UK/Ireland, if doing so would cause the total number of days spent in any country of the Schengen zone to exceed 90 days in the 6 months following the date of first entry.