Ireland is a small country, so you can easily see a lot of it in just a few days. However, if you want to really experience all that Ireland has to offer, then you'll need at least a week.
If you're short on time, then you can focus your trip on one specific area. For example, if you only have a few days, then you could focus on exploring Dublin and its surrounding areas. Or, if you're looking to experience Ireland's stunning scenery, then you could spend a week driving around the Ring of Kerry.