RubyJobs (http://www.rubyjobs.ie/jobs) sometimes has startup positions listed but your best bet is to just pop down to a Ruby meetup (http://www.rubyireland.com/) cos there's where all the startup peeps hang out. The first line of dialogue with practically anyone at these things is "Are you looking for a job" - but obviously the more well known you are in the community the more often you'll be asked.

My experience is relatively limited (about 5.5 dev years) but once you've a good reputation then you'll never have to look for another job - it'll be lined up for you. Until the bubble bursts that is!

I also know a good recruiter (one or two exist!) who does work for the odd startup if you're interested in taking that route. Email me (ger [at] getchoocreations.com) if you're interested.