One SEO company I know to be good is http://aonach.com/ . I work in the area but for a private company so can't offer my own services ! I've worked with these guys before and know their work is good.
A few others I know of are
redcardinal.ie
online-marketing.ie
The ones I mentioned above should be among the more reasonable. There are some larger agencies that charge way over the odds and frankly I have no idea what for other than theri ability to gauge how much cash they can extract!