As long as your Irish visa is valid, you can enter and leave Ireland as many times as you wish with a multiple-entry tourist visa. A multiple-entry visa, on the other hand, is only provided in certain circumstances, such as if you have previously acquired Ireland visas or if you need to make repeated short journeys, such as for business.
Anyone visiting Ireland as a tourist or guest is only allowed to stay for 90 days. This includes both EU/EEA citizens and non-EU/EEA citizens who are not required to have an Ireland visa.
If you intend to stay in Ireland for more than 90 days as a non-EU/EEA national, you must apply for an Ireland Long Stay Visa and obtain a Residence Permit.