Ireland can be a pretty cold and rainy place throughout the year, so much of their fashion and dress is based on that fact. A raincoat, although maybe not fashionable, is one of the most important pieces of clothing that a person can own, and comes in handy all the time.
Depending on the location, the fashion trends will change. In many of the coastal villages and fishing towns it is very common to see sturdy boots, comfortable pants or jeans, and something warm to wear on top of it all. In areas like County Wicklow, there are lots of sheep which have wool that is spun into some of the best sweaters around. So in the colder rainier villages, a good warm sweater is easy to find, and for good reason. When cotton gets wet, it tends to make a person colder. Wool, on the other hand is resistant to water, which keeps the wearer warm, so it is quite popular in Ireland. Lastly, to keep the head warm and rain out of the eyes, a flat-cap might be worn by many of these Irish folks. The flat cap used to be called a “bonnet” until about 700 years ago, and now they are easy to find all over the US, mostly thanks to Irish influence.
When not in a coastal village, it is likely that the fashion is somewhat similar. Like Irish food and Irish customs, they often dress plainly, without unneeded frills and accessories. What is woolen and warm will usually do the job, and earth-colors are the most prominent shades. There are lots of deep greens, soft browns, and grays. It would be someone out of the ordinary to see a very brightly colored flat-cap, for example.
In Dublin, Cork, or Belfast, for example, a greater variety of fashions might present themselves. In big cities in general, it is much easier to find a wide variety of styles and dresses. The average Irish fashion for modern clothing is quite similar to what can be found in Western Europe of the US. Jeans and T-shirts are the standard among most young people. The range of Irish fashion is as broad as any country. Most people probably dress somewhat simply, and other people are more interested in wearing intricate and flashy clothing.