Budgeting question--price of gas in Ireland

Google "Gasoline prices Ireland" and "mpg ford fiestat"

The British AA has a page with Fuel prices across Europe, including Ireland.: http://www.theaa.com/driving-advice/driving-costs/fuel-prices
Then click on the latest month (currently December 2018).
Ireland: Unleaded petrol €1.40 per litre, Diesel €1.32 per litre

A bit of Googling got me 5.8 Litres per 100 Km, but that of course depends a lot on the roads and your driving style.
Beware of any figure quoting MPG. That is probably Miles per British Gallon.

A Ford Fiesta is not a small car, it is mid-sized. It has grown over the years (as have many other models, notably the Golf).

They don't call it "gas" in Ireland, it's petrol. "Gas" is the stuff that comes through pipes into your house to run your cooker or heating; or, in the context of vehicles, LPG (Liquified Petrleum Gas), but few cars run on that.
You need to Google "petrol prices Ireland"

We had a diesel car for the first time ever. I couldn’t believe how long a tank of diesel lasted. Over a week! Drove from Shannon to Dingle to Killarney, drove the Ring of Kerry, and then back to Shannon.

I wouldn't expect you to spend over €100.

It's not Ireland, but when we were in Germany in fall of 2017, we had a diesel car the size of a Ford Focus. We drove 1000 km in a week, and filled up for 60 euro upon return of the car. That was all we spent in the entire week.

At $1.14 = €1, a gallon of diesel (at €1.32 per liter) would be about $5.71. If the car got 30mpg, then you would use 20 gallons to travel the 1000 kilometers, meaning the fuel cost would be about $114.