Ah, well event advertising is a little different than product advertising because you've got to get a bunch of people in a single place at a specific time. There's a deadline, and webcomic promotion doesn't work so well with events since it takes years to build a reasonable audience. (I work in a marketing department that deals a lot with events, so I've got a pretty good idea of how they differ.) Events advertising and webcomics are kind of on two opposite ends of the spectrum in terms of promotion.
One thing to remember about events is people need to be reminded, and reminded frequently about them, getting increasingly frequent towards the actual event day. In today's day and age, as far as event publicity is concerned, you need to collect people's information, either email, facebook or twitter, to remind them of your event. If you don't have a website, a facebook page, and a twitter account for your event, you are shooting yourself in the foot. You should be clear about what your event is, and why people should come. Be concise, short, and sweet. On your website you should have a mailing list sign up and probably a forum so people can ask questions and stay informed. The mailing list will allow you to send out updates and newsletters as well as reminders as the event draws near.
As to where to find an audience, I'm not sure about your area, but you'll want to promote most heavily in the areas closest to the event, probably within 50KM. No one outside that area will probably come, especially if your event is new and relatively unknown. I'd suggest looking at other events in the area, such as renfests, or oither cultural events where you could pass out business cards or fliers encouraging people to go to the site and sign up for a newsletter, or even better, run around with a laptop or Ipad and sign people up on the spot. You should appoint someone in the event staff who's good with twitter to start building an event twitter account, follow other celtic resources, spew celtic facts and interesting articles, get people to follow you, and follow other people who are into celtic stuff as well as other event co-ordinators. This will get you some cross traffic, and promote the facebook page through a like button on the website, and encouraging people to like the page on twitter and in email while you are leading up to the event. You may also want to search around the web for celtic resource or community pages and make them aware of your event. In a non spammy way of course.
Even if your event isn't heavily technologically related, you need to use technology to promote it. Although don't discount clever paper promotional pieces and reaching out to the communities that already exist which may already have an interest in what you are offering.
You want to build hype and excitement around your event and the great things happening there. Focusing around a webcomic and sort of hoping people will come to an event is not really the best use of your time. Although if you can find an EXISTING celtic themed webcomic and get them to maybe mention your event and plug your event webpage, that might be a better use of your time.
