There are signs all over the city that basically say one or the other. What the vote is all about is something called the Lisbon Treaty. Apparently it is a 500 page treaty that was signed in Lisbon (surprise, surprise) on December 13, 2007.
I am no expert on this thing and I'm sure I know less than the average Irish citizen, but it's complicated in that they aren't giving a lot of information. They do provide the link to read the entire treaty, but even a lawyer would get bogged down in all of that legal mumbo jumbo. You'd be asleep by the end of page 2. They are also providing small brochures in the mail to attempt at a layman's explanation, but it still doesn't make a ton of sense. My personal opinion on the reason why is that they are asking for a yes vote and no one really knows what the yes vote will bring. But they know what the no vote will bring - status quo. A quote I like to use in many situations - it's easier to "stay with the devil you know."
It seems that the treaty will attempt to further reform voting procedures. The "yes voters" say it will make a stronger European Union. I would think that most people who vote no will do so because there isn't enough information as to why it's a good idea to vote yes. There's a ton of reading to do out there, I'm sure, so I'll update once I have more time to learn about it myself. In the meantime, I've included a few informational links.
10 reasons to vote YES: http://www.ibeclisbon.ie/
Sinn Féin says vote NO: http://www.no2lisbon.ie/