each stay is max 90 days, so when is it ok to start the next stay.
How long do you have to be outside Ireland before you can start a new 90 day visit ?
With Schengen Visa, its 90 days in 180 day period. simple
But I cannot find any Irish rule other than each stay should be no more than 90 days,
the following very Irish meaning of don't push it..
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.