Ireland North to South Itinerary ?

It would help to know when you are going. If it's in summer, try university accommodation.
My friend and I stayed in the Business School student housing at Trinity this summer, and we were very pleased. We each had our own large en suite room with a twin bed just down the hall from each other in an apartment with a shared kitchen and separate "lounge." The lounge was comfortable if a little sterile. The location is unbeatable, of course, and we were right at the edge of campus, with great access to airport transportation.
Near Portrush, Coleraine has a university that I believe offers student housing rental in summer. That said, I would stay in Derry way before I would stay in either Portrush or Coleraine. In Derry, the same friend and I stayed in two rooms at Quay8, a hotel with very small en suite rooms with, as I recall, double beds, inside the walls. We are budget travelers and were very pleased with our accommodation. We didn't have a car, so I can't speak to the parking situation.
Re: Donegal, I hope you are referring to the County, not the City, because there are far better options in the area. We LOVED Killybegs and wish we could recommend the place we stayed, but it only had two beds. Driving down from NI/Derry area on the N15, I think it's called Barnesmore Pass? You wouldn't think you were in Ireland. We were on a hurtling bus and the views were still amazing. I would love to see it from a car that can stop.
With a car, of course, you can stay nearly anywhere along your route, so be sure to check AirBnB and the other major booking sites. Anyplace worth staying will make clear how many beds there are and in what configuration. If you don't need separate rooms, only separate beds, traditional hotels and B&Bs usually have twin rooms. You just have to double check that that's what you're getting.
For outdoor exploring, the North wins for me, hands down. I would shift some days if possible. The Sperrins Mountains are beautiful, and the Wild Atlantic Way was created for a reason.
Have a wonderful time!

Several years ago, 3 friends and I stayed at the Castle Hotel in Dublin in a quad room and each had our own bed. Just to check, I looked at a random date in May and a quad was a possibility. But, of course, I don’t know your dates.

Edit to add: Like G3rryCee, I have also stayed at Trinity College in two rooms with twin beds. If your timing works, it gives you the advantage of two bathrooms, as well as being centrally located..

Few notes I want to bring to your attention (for reference I am from Northern Ireland)
1) the N15 is in Ireland not in the UK (Northern Ireland) our roads are designated A and B roads then Motorways
2) Northern Ireland like the rest of the UK drives in miles not kilometres as Ireland does
3) currency in Northern Ireland is pounds not euro
4) all visitors from 2025 January will require a electronic authorization to enter the United Kingdom this applies to everyone except British citizens and Irish citizens this can be applied for on the UK government website (www.gov.uk) inputting "ETA " despite the border being open cross it without one you'll be liable for immigration offences and deportation out of the UK and banned from entering so please get one they will cost £10 pounds

I'd not recommend Portrush, at least from what we saw in 2023. It is large number of boarded up buildings, very few open restaurants, and lifeless pubs, all with a large permanent carnival in the middle of town. And then there was a car race that closed many of the roads, although that was just bad timing. The Prospect B&B we stayed at (the only inhabited building on the street) had a great view, good breakfast, and friendly hostess. She explained the recent UK government restoration grants were larger for derelict buildings than for remodels, so private investors had snapped up all the properties and let them deteriorate. We stayed there to access Rathlin Island, Causeway, etc. and doing it again, I would stay in Ballycastle, Portballintrae or Bushmills. Our next trip has us staying on the Antrim Coast instead (probably Cushendall).

The ferry from Kilmer to Tarbert will reduce your drive length, but won't save any overall time unless there's traffic in Limerick. Still a nice route and ferry is frequent, at least in May.

I always try to book accommodations 9-12 months in advance. Our 2025 trip still has two open bookings because they don't take reservations until Jan 1, and I'm nervous.

I would love to know where these boarded up buildings are i live ten mins away from Portrush

I apologize to Portrush if things have changed in two years. On Lansdown St. which is the "scenic" stretch, I only see half a dozen boarded up buildings on 2024 Google Maps imagery, so there's been some restoration or my memory is bad. As far as the amusement comment, there is a big game center and the day we arrived, there were hot-air balloons tethered and bright lights in the area and several patrons in a bar and restaurant complained that the business had hollowed out the old town center.

Regarding your trip from Galway to Dingle: if you go via the direct route (the motorway to Limerick and the N21 from there) it's only about 4 hours by car ... depending on traffic getting thru Limerick.
If you want to make it a meandering, all day, and much more scenic drive I'd suggest jumping on the N67. It's a very pretty drive on a sunny day, with potential stops along the way at the Burren (recommended) and the Cliffs of Moher (not). A possible overnight stop would be the pleasant little seaside village of Kilkee . There's a very nice (level) cliff walk there that features spectacular views of the coastline without the crowds of Moher. Nice stretch of sandy beach there too.
If you're up for an off-the-beaten-track excursion the peninsula just south of Kilkee is quite pretty too ... all the way down to a scenic lighthouse at the end of the road.
From Kilkee it's only about a 20 minute drive to the Kilimer Ferry across the Shannon, and from there it's about 2 hours into Dingle. along the N86. If you're particularly adventurous you can swing to the north and come down into Dingle via Conor Pass thru the mountains, but that's not for the faint of heart if the weather turns sour.
I think 4 nights in Dingle is about two too many to satisfy your curiosity about the place. Better to take those 2 days and maybe extend in Connemara or explore further south towards the Beara peninsula.