Pros
Very easy-going people, everyone very friendly and collaborative. People are humble and you have help any time you need. Also there's not excessive rules or procedures, the way you wouldn't find in a bigger company... Definitely the place to be if you are tired of the bureaucracy of the big corps. Free lunch every day, free snacks, great office located in the city center in Dublin; very useful tool that will help not only those most expert project managers but also whoever who is managing a project and day-to-day tasks. Ping-pong table, company events every now and then (kick off in Mexico last year!) - you get a Mac computer which is very good to work with. Possibility to work from home, flexi-time (as long as you perform well and you have your manager's trust, you can build your timetable...).
Cons
Career path not determined yet but getting there - the good point of being a startup is that room for opportunity happens and you can become another role working on it... Easier that in big corporations where change takes longer to take place.