I am a commissioning editor in an Irish publishing house so hopefully I can help with this.
New authors do not automatically require an agent, most Irish publishers will accept unsolicited manuscripts directly, however it's advisable to give them a ring first and check how they wold like it submitted, and more importantly whether or not they publish your type of book. They will probably ask for a few sample chapters, a synopsis and a cover letter.
There are three agents in Ireland; Faith O'Grady, Jonathan Williams and Marianne Gunne-O'Connor. They all have offices in Dublin. Obviously Irish authors are also represented by UK agents, but I don't have details for them. Try
The Writer's and Artist's Yearbook.
In Ireland the main fiction publishers are The Lilliput Press (literary fiction), Mercier Press (general fiction) New Island Books (general fiction, some poetry) and Poolbeg (they publish a lot of chick-lit type stuff). You should check
www.publishingireland.com for a complete litst of Irish publishers and links to all the websites. The Irish Writer's Centre also have a website that gives you a guide to contracts and how to deal with publishers (we can be a tricky bunch!).
Do bear in mind that fiction is one of the hardest types of writing to get published, and 99% of submissions from unknown authors are rejected, mainly because of bad writing. When submitting your sample chapters and outline include a cover letter giving details of any other writing you have had published (e.g.short stories in magazines, literary journals, etc).
An average work of fiction sells about 2000-5000 copies in Ireland, so if you find a publisher don't expect to make your fortune through writing. You will probably be offered a very low advance initially (€1000 maybe) and then a royalty deal (10-15% of publisher's receipts (in other words a percentage of what they get from the bookshops. Bear in mind they sell the books to the bookshops at approx 40-52% discount, so if the book costs €12.99 in the shops, the publisher gets about €7 and the author about 80c).
I'm sorry this all sounds so negative, but I'm afraid that's the reality. If you have any more questions I'd be happy to help.