Ireland has 3 festivals in this guide, spread across 3 cities. They run in July and August, and festival-day temperatures range from 18°C (64°F) to 19°C (66°F), with nights down to 11°C (52°F). That range is the reason there is no single "Ireland festival outfit". The answer changes by month and by site.
3 carry a high rain risk, which makes waterproof footwear a requirement rather than a precaution.
The ground matters as much as the sky. You will find city park, open field and forest, which puts waterproof boots at the top of the list for most of these events. 2 of the 3 have camping on site, so outfits have to survive living in a bag on the ground for several days.
Stylistically the crowd here leans boho, indie, camping and streetwear. Electric Picnic is the largest of them at around 70,000 people, and it sets the tone the smaller ones react to. None of that is a dress code. It is what you will see, which is worth knowing if you would rather blend in or deliberately not.