Escamillo,
You ask what to drink besides Guinness in an Irish pub. Well, there's Kilkenny, a creamy brown ale but rather bland. There's Murphy's a creamy black stout, but rather bland. Irish lagers are plain awful. Stick to the Guinness, it's your only man. And leave the whiskey (note the spelling) till the end of the night, when you're already under the table. Don't accept an Irish coffee if it's served with a straw, and make sure they use fresh cream.
Irish stew, made with lamb, onions and potatoes, can be very good.
Concerning etiquette: treat your Guinness with due respect. Wait for the head to fully separate from the black body of the drink before tasting. Don't pour it over anyone else. Only English football hooligans do that.
It's normal to order at the bar and pay for each round as it's served, especially if they're busy.
Once you've made your order, make a little room for other people trying to get to the bar.
If someone's carrying drinks back to a table make way for them. Spillage of good Guinness is a serious matter.
Be aware that the Irish are not English, or even British. And tell them you know all about Admiral Brown and Bernardo O'Higgins being Irish, and about how most Irish migration to Argentina was from certain parts of Westmeath and Tipperary, where the landlords paid their passage in order to be rid of them, and how Velez Sarsfield is named after an Irish man, and the Christian Brothers came from Ireland, etc etc. Irish people are easily pleased by people showing some knowledge of their country other than "IRA bang bang".
But I'm probably putting you off with this sort of unnecessary advice. The whole point of Irish pubs is they're meant to be warm and welcoming places. And they are there to please the customers, not the other way round. I hope you have a great time.
Art O'Malley