Spring's got budding flowers and baby animals.
Summers got looooong hours of sunlight and higher prices and some locations have a lot of tourists.
Fall usually has good temperatures with some sites cutting back on open times.
Winter doesn't has very short days, so pubbing practically becomes a requirement :)
Here's a post you might find helpful http://irishfireside.com/2012/03/04/when-is-the-best-time-to-visit-ireland/ almost 6 years ago
Outside the school holidays in mainly July & August will probably be quieter and less expensive. September is a good month before the cold months of winter start to set it. Prices are cheaper & there'll be less noisy kids & crowds if that bothers you. Obviously it all depends on what establishments you stay in. Late Spring.early summer is also a good time i.e. May & June, even as early as April. Before that it can be a bit cold if that bothers you. almost 6 years ago