Weather - it will depend on the time of year. We went in April and it was in the low 70s the whole time, which is out of the norm.
Things to see - Cliffs of Moher, Dingle Peninsula
What to pack - Layers! Also in my experience Ireland was very casual, so jeans, t-shirts, and my regular running shoes were appropriate.
Currency - they use the Euro, and everything was expensive, especially because of the added VAT. I got all the receipts to get that back at the end, but for various reasons it ended up not being worth it to try to get it back. They use chip'n'pin cards there, where we use swipe cards here, but I didn't encounter any place that couldn't whip out a swipe machine for me. One place a younger teen cashier was confused, and I told her to just swipe it and follow the instructions and it was fine.
Food - sticky toffee pudding!! I did a mostly traditional full Irish Breakfast and had white and black pudding. White pudding was awesome! Black pudding was strange, but OK. The grossest part of the whole breakfast was actually the mushy broiled tomato. Cadbury Egg McFlurry's at McDonald's are a huge disappointment. Buy the Cadbury Egg candy bars instead. And Happy Hippo Kinder Biscuits. Yum!
Places to stay - Can't help you, we stayed with a friend in Killarney, and then random hotels by the airport when we got stranded.
Etc - Don't be afraid to rent a car and go driving! I sprung for an auto because I kind of suck at standard, and it was totally worth it. I would recommend GPS, but I will warn you that it was mostly useful in getting us to a city, but not necessarily to a specific place in said city.
I went last year with my mom, who has always wanted to go. We went the first ~12 days of June. Weather was cool, but mostly sunny. I wore skirts/skorts and long sleeves mostly.
What do I need to know about: Weather Be prepared for it to rain! Temperatures vary with time of year.
Things to see We did not go to Dublin; we flew in and out of Shannon and did western Ireland. My favorites were Connemara (beautiful scenery), Dingle, Cliffs of Moher, Doolin, and I also really enjoyed a brief afternoon in Cobh. Kinsale was meh. We skipped the Ring of Kerry in favor of the Dingle Peninsula.
What to pack Comfortable shoes. Relatively casual clothes. A good camera. An outlet converter for any electronics (charging camera battery). Note, don't try to use a converter with a hair dryer!
Currency You generally get the best exchange rate by just w/drawing Euros at ATMs, so that's what we did. Bring at least one credit card that does not charge foreign transaction fees, like Capital One's.
Food Restaurant fare is excellent, but pricey. Otherwise, there's lots of pub food. I ate a lot of seafood, not that there aren't other options. I really enjoyed the Irish breakfasts at B&Bs. Between breakfast and dinner out, my mom & I only really ate 2 full meals/day, with a snack in between. Calvin wouldn't have gone with that though if he were on the trip.
Places to stay Depends on the city. Pick the city, then research accommodations.
Etc I highly recommend Rick Steves' Ireland travel guide book... and my mom and I read them all! Planning this trip was our activity together while she was going through chemo, we went as soon as she was ready afterwards. I appreciated that he gave some opinions about what to do with a certain amount of time, and prioritizing sites, whereas other authors say "well if you're in this county, here's the best thing to see." There isn't time to hit every county, so I didn't find that so helpful.
I think renting a car is the way to go to get around. It's an adventure on its own! Plus it's the only good way to get where you want, when you want. If you can't or don't want to drive stick, a manual will cost more and need to be reserved in advance. We both drive stick at home so we gave it a whirl. Left hand shifting isn't so bad. We bought a map and had a GPS. The combination was helpful to navigate.
I also really liked staying in B&Bs. I had some conversations I really enjoyed and remember with B&B hosts.
Ireland is fantastic...you will have a great time!
I definitely recommend renting a car and staying in B&Bs. It's the best way to see the country. We went in June 2007 and it was unseasonably warm - I would recommend packing layers because by the end of the trip it was raining and chilly. The Rick Steves book is really good...we saw pretty much every American with it.
I highly recommend going to the Dingle Peninsula. It's gorgeous - we did the bike ride around the peninsula from the RS book and it was the highlight of the trip. Killarney National Park is also really nice and Kilkenny is a cute little city/town.
Weather: We went in Sept and had great weather. The highs were in the mid-60s and it only sprinkled one or two days the whole trip.
Things to see: Everything! In Dublin, Kilmainham Gaol, the Chester Beatty Museum, the Book of Kells at Trinity College, Christchurch Cathedral, and St. Michans. Elsewhere: Glendalough, Kilkenny (St. Canice's cathedral), the Rock of Cashel, Cahir Castle, Drombeg Stone Circle, the Dingle Peninsula, the Cliffs of Moher, Galway
What to pack: clothes! It really depends on what time of year you are looking to go
Currency: A few places take credit cards, but I'd recommend just pulling a bunch of Euros from an ATM when you get there.
Food: Try the locals ales-- there are some great ones. And I was such a huge fan of Guinness there. Here, it's not as good, but give it a try at a pub. Also, you can't go wrong with pub food, it's all delicious. I was also a much bigger fan of the black pudding at breakfast than I thought I would be.
Places to stay: If you decide on what areas you want to visit, I'll be happy to offer any rec's we may have had for those places. We did a couple boutique hotels, a couple B&Bs, and one castle hotel.
Etc: Don't be afraid of the driving! It's not as bad as a lot of people made it scene. So definitely rent a car and check out some of the country side. Also, get a heritage pass if they are still offered. I have forgotten how much it cost, but it got us BOGO admissions to several sites and paid back its cost in no time. I recommend Frommers and DK Eyewitness travel guides. And if you're interested in reading our blog from our trip, here's the link: travel-jc.blogspot.com/search/label/ireland
Weather: We were there in August 2010. I wore jeans pretty much every day, my sister wore skirts and was happy. It rained a lot but it was never a downpour.
Things to see: Cliffs of Moher, town of Kinsale, Guinness Storehouse (although I personally wouldn't spend more than a couple days in Dublin), Ring of Kerry drive, Muckross House, Trinity College (Book of Kells)
What to pack: hooded rain jacket/windbreaker (we didn't bother with an umbrella), a couple pairs of comfortable shoes, dry socks
Currency: Euros. We had no problem using our debit/credit cards for most things, though.
Food: Beef/lamb and Guinness stew was served at most places and was always tasty. Seafood was good. The ice cream is amazing. Chocolates were yummy (Cadbury especially). Irish breakfasts. When we were in Kinsale we tried to go to Fishy Fishy, which is supposed to be amazing, but it was closed when we got there.
Weather We went in late March of 2002. It was chilly but sunny, which made for great traveling weather.
Things to seeI can't think of anything to what everyone else has mentioned. It's just such a pretty country.
What to packI brought sweaters and a light winter coat and was fine.
CurrencyWe did a package with B&B vouchers, but if you're not going that route and still want to stay in B&Bs I'd check and see whether they offer a discount for paying with cash, which isn't uncommon in Europe.
Food We made a point to drink Guinness every day, and I was very impressed with the food in general.
Places to stay As I said, we stayed in B&Bs that accepted our vouchers. My biggest tip for B&Bs is to ask your host for a good one in the next town you're going to, then have them call to see if they can accommodate you. The only place this didn't work great was Dublin and we ended up in kind of an outer neighborhood.
Travel is fatal to prejudice, bigotry, and narrow-mindedness, and many of our people need it sorely on these accounts. Broad, wholesome, charitable views of men and things cannot be acquired by vegetating in one little corner of the earth all one's lifetime. Mark Twain
OP- I don't have responses to your inquiries, but did want to mention that there are a ton of travel packages for Ireland right now.
We booked a package for September with Scepture Tours vis Travelzoo. For $1,150 a person we have 7 nights hotel (1-Kilkenny, 2-Blarney/Cork, 2-Limerick, and 2-Dublin), direct flight from Ohare, and a rental car. We paid for insurance and to upgrade the rental car from compact manual to full-size automatic (we have 4 people sharing it). That's cheaper than the price of the flight outright from the airlines website.
I'll post a recap after our trip, but reading all of the responses has be so exited!
Ireland uses the Euro. Northern Ireland uses Pounds.
I recommend getting a car and driving around the country. I did a road trip a few years ago from Belfast to Donegal and down to Galway. So many gorgeous towns and countryside along the way. Galway was a great base as lots of places were within an hour or two drive. We stayed at little B&Bs along the way.
Dublin is great if you want more city life than countryside.