I'm interested in starting an ecommerce/ community website, how do I identify my market?

[Irish]

from the research I've done, no such direct competiton exists... so my market potential could be very broad, despite having something of niche at the same time. I'm attempting to write a business plan, but am stuck on the market research because I'm unsure of where my business would fall. Any ideas on how to narrow the field would be helpful.