Depending on where you came into the pagan paradigm will certainly color your views on the type of ritual garb you should have or whether you should wear ritual garb at all. The idea of specific garb for ritual has always been to set the mood and bring you into the correct frame of mind. To some this can go from the simple one piece pull over shirt all the way up to the elaborate wizarding costume. Some folks even prefer skyclad worship to anything at all. While we have no real proof that our ancestors practiced skyclad in their rites, it is a modern tradition that is popular with many. But as a practitioner of Celtic spirituality do we need special gear to wear in our rituals? The short answer is that depends on the individual. The longer answer...
I am a firm believer that you do not need special garb to worship in. If you would like it, by all means wear it. No one should say that you should not wear it nor what you should or should not wear, especially in your own private rituals. I will touch on public ritual here though. In a public setting I do not feel you should have to wear special clothing or jewelry if you don't wish to. One Druid group that I am a part of states that they feel that people attending ritual should change out of their everyday wear and put on something to help induce the mood. The thing is, our Celtic ancestors probably did not wear special costumes in order to worship or to go to public holy day events. They did, from what we can read about in surviving texts and legends, wear their best clothing to these public feast days. Sort of the equivalent to wearing one's Sunday best when going to church. The people who held items of jewelry against a debt were even obligated to give those items back to their owners for feast times so that they would not loose face among their tribe.
So what does that mean for us in our world of today? Well as I said I do not believe you need special clothing to worship in, either in private or in public feast day times. It would be good if you wore something clean and nice as you would for any religious event. Something that you could be proud to show up to an event in. I am not by any means suggesting formal evening wear unless the event happens to require it. Remember many times we worship outside in nature so dress for the environment of where the event is taking place. But don't feel pressure to wear something fancy either. In this day and age money can be tight for people and dressing for a religious ritual should not be something that breaks the bank. Nor should it be something that you should be afraid to attend if you only have a t-shirt and jeans. My best advice is to wear something you are comfortable in. Wear whatever jewelry or clothing that you feel your best in. Feeling comfortable is a big key to getting in a good head space when communing with your kindreds. So if you feel better in your "street clothes" when worshiping then by all means wear them. Be yourself.
Does that mean you can't wear fancy gear to ritual? Not at all. One of the biggest exceptions to the idea of wearing what you feel best in, is ritual leaders. For those performing ritual it is really a good idea for you to stand out against the rest of the people at the ritual. Whether it is ritual robes like those of the Druid revival:
Or something a little more authentic to the Iron Age:
To something even a bit more modern:
Anything that can help a ritual leader stand out is a really good idea. What this does is it allows people attending the rite to focus on the people doing the ritual. I have been to rituals where those leading did not change out of their regular clothes and it was hard to tell who is who after a while. Sometimes going the extra mile when leading ritual can make for a more powerful experience for those who attend. Drama is always increased when you have something interesting to focus on while the the ritual leader is invoking the Gods.
Does that mean that non-ritual leading participants/congregants cannot wear ritual garb? Not at all. As seen above there are plenty of options for Celtic ritual wear that one could choose from. And upon reading up on the subject of Celtic dress in the ancient times you can find out all sorts of interesting information that you can apply to your ritual garb. For instance it is said that depending on one's status in ancient Celtic culture would depend on the amount of different colours one could wear. In Luke Eastwood's "The Druid's Primer" he mentions:
"The kings or queens wore red or crimson and their offspring could wear scarlet, blue, or purple. Nobles could wear green or brown. It is said that royalty might wear up to seven colours, the Ollamh or Bard six colours, chieftains five, great warriors four and so on down to commoners and slaves who wore one colour only. In Scotland, the royal tartan had seven colours and only the king or queen could wear purple in their tartan. This colour system is the basis for the system employed for the tartans of Scottish Clans, used in kilts and cloaks today."
These are only a few ideas that you can look to when creating ritual wear. There are other ideas you can use to add to your garb or even to your own personal identity as a person following a Celtic spiritual path. Jewelry, specifically Celtic style jewelry can help. Items such as a torc can be something you wear both to ritual and outside of ritual to keep that connection to your kindreds.
Ritual wear should be a personal choice. It shouldn't be something you have to do when worshiping the Gods. Just remember that what is important that you feel comfortable when attending ritual. If you want to wear it then do so. If you want to wear a t-shirt and jeans, then do that too. What is important is creating a relationship to your kindreds. In the end it is that relationship that is what is important, not what you are wearing to ritual.
-Eastwood, Luke. The Druid's Primer (Moon Books)