Consecutive Schengen and non- Schengen stays

Ireland is not in the Schengen Area. Days in Ireland do not count as days in the Schengen Area (and vice-versa). But note:

• The day you travel from Portugal to Ireland will be both a Schengen day and a day in Ireland. Arrival and departure days count as full days.
• It is normally not a good idea to go up to the 90 day limit. Anything can happen to cause a delay (illness, cancelled flight) which could put you over the limit.

I can't think of any unless Ireland refuses you entry? Is there any reason they would?

Don't forget that both entry days and exit days count towards the 90 days. If your last full day in Portugal was number 90, the day you travel to Ireland would be 91 and you would be an overstayer.