Yes, the Republic of Ireland uses the Euro. Most large purchases can be done via credit card but you will need some cash for gratuities and small purchases. You can obtain Euros ahead of time from your bank at home, or get them from an ATM or currency exchange on arrival. (Notify your credit card and bank that you are traveling, and check with your bank whether you can use ATMs internationally). There is a currency exchange at the Dublin airport, but Irish banks do not offer currency exchange without an account held there.

This is really a personal preference. Most purchases can be done with credit cards (check with your card to make sure international purchases will work and whether they need to be notified of international travel). Some shops have minimums to use credit cards, so cash is good to have if you're just buying a coffee or soda etc. You'll likely want to use cash for gratuities as well for hotel staff, restaurant staff (can put gratuities on credit cards at restaurants but it is not as easy to do as it is in the States).

The majority of Ireland has become accustomed to tips (thanks to being a huge desitnation for American travelers).
- Restaurant staff do make minimum wage there though, unlike many places in the US where they make significantly less. 10-15% tips for meals out is customary for good service.
- Hotel housekeeping customary gratuities are similar to the US, a few euro per day is generally good, depending on whether you have your room made up each day or not.
- Drinks at a pub... somewhat complicated. If you're ordering a drink at the bar and taking it to a seat elsewhere, no real need to tip unless you feel like it. If you're sitting at the bar for a few drinks, it's customary to leave something but there's no exact science.
- Coach Driver: I’ve included a cursory gratuity for our driver, but if you feel he does a fantastic job you’re of course welcome to add to that

You will need a three pronged "UK style" converter for anything you want to plug into the wall. FYI, most converters say they are not suitable for heat appliances like hair dryers (hotels have their own), curling irons etc. Be sure to read what you're purchasing and make sure it fits your needs.

Check the weather report before you pack, and then expect it to be entirely wrong. The only predictability about the weather is that in August you can expect anywhere from 50-80°, sunny heatwave or lashing cold rain, lovely picnic weather or wild wind. It's not unheard of for it to happen all in one day. A raincoat may come in handy, umbrellas are generally useless since when it rains it's rarely vertical.
Layers, layers, layers are the answer. Generally speaking, people dress a bit nicer in Ireland, which doesn't mean that you need to, but depending on your personality you may feel underdressed in gym-wear or Tshirts and sneakers.
Because it typically does not get very hot, many buildings do not have air conditioning. Hotels without AC can provide fans, if necessary and have windows that open.

Most cell phone providers have international calling plans, check with them directly. Whatsapp is a free app that you can use to text or call anyone else with the app over wifi.

Get on to your new schedule as soon as possible. It is really difficult to stay up all day when you arrive in the morning, but it is the best way to do it. If you need to nap, keep it very short and as early as possible. Drink a ton of water and be outside in natural light as much as possible and eat meals on your new schedule. Do not keep your watch on your "home time." Do not think of your home time as "real time."

- Many hotels do not provide washcloths, as they are considered a personal item. 
- Have a copy of your passport and phone numbers for your credit cards and keep them in a separate place.
- List of important medications and dosages in case yours are lost (Do not put any medications in a checked bag, and make sure prescriptions are in their original containers).

Taxis are readily available everywhere. There are some taxi apps that work well such as FreeNow but any hotel can call taxis for you and  there are taxi ranks (stands) located throughout cities in popular areas. Uber does exist in Dublin and some other places, but essentially just connects you to taxis. For getting around Dublin, if you don’t want to use taxis, there is a tram called the LUAS.  A round trip  ticket for the central zone (most places you'd want to go) is €4 and can be purchased at machines at any station. Keep your ticket on you, though its likely nobody will check it. Unlike trains in the US, you have to push a button to open the doors getting on and off.