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.