Renting a car and driving alone wasn't a big deal for me. I have experience on small country backroads in the U.S., so the narrow roads weren't too big of a transition, although there was some adjustment.
It is more expensive without someone to share fuel costs and the rental costs, but it is more convenient and allows you to set your schedule better. I enjoyed lingering in places where I couldn't have if I had been on a tour.
Trying to navigate and drive is the biggest challenge, so I downloaded Google Maps into my phone before I left and then used the offline voice navigation feature while driving. It made a big difference. Many of the streets are not well marked for drivers, so trying to use a map or written directions and find a street sign will also driving would be tough.
I wouldn't try to drive in Dublin, but renting at the airport was easy, and getting out of and back to the rental lot was easy.
I enjoyed staying in hostels when I was younger, but need a bit more privacy and convenience now that I'm older. I used Airbnb and regular guesthouses and B&B's, and met a number of pleasant people that way. Some of these are pretty inexpensive (not as cheap as hostels, though).
I would encourage you to stay after your friends leave and enjoy extra time in the country. I frequently did that in my 20's because my friends had less vacation time than I did. It's great to travel with friends, but then have some extra time on your own. Both experiences are valuable and have different advantages, and I haven't met anyone who has said, "I wish I had spent less time travelling when I was young" or "I wish I had spent less time in Ireland."