Cork-Belfast will be much closer to the estimate given by the online calculators as for most of the trip (from Cork city to the border) you will be using good motorways (M8/M7/M50/M1) where you can go 120kph (unless posted otherwise).

Traffic can be a little crowded on that short leg around Dublin on the M50. But you should familiarize yourself with the options how to pay toll for the M50.
eflow.ie

There are a few "toilet stop" options with also a bit to see near the motorway, e.g. the Japanese garden near Kildare.

Are you taking a rental car across on the ferry? IMO that isn't a great idea (and is very expensive).

Instead of dashing the length of the whole island why not fly from Cork to Scotland and save an entire day?

Thanks Cowbooy1968 & janisj for your advises!
You have any idea how much of toll charges should we be looking at?

janisj, we are taking up a rental car but it will be within Ireland and Norther Ireland only, we plan to pickup the car in our 3rd day (1st 2 days in Dublin will be on foot/public transport) in Dublin city and return the car 3 days later in Belfast or Larne.

Not sure if this is accurate but I saw online the ferry is only about 2hrs ride. We will not take the car across on ferry, instead we will rent another vehicle in scotland and return in London.

We were thinking it would be nice change to ride on a ferry instead of flight as we are traveling from SG to Lon (14hr flight) & Lon to Dub (1hr+) and return Lon to SG (another 14hr)..

You find info re. M50 toll on the website I mentioned.
It should be around €3. But it's not so much The amount but how to pay for it.

Your plan to hire an Irish-plated car in the Irish Republic and return it in Northern Ireland (part of UK) will alsmost certainly result in high one-way drop-off fees.

Northern Ireland is The ROI like Canada is the USA. A car rented in the South returns to the South either by you or at your expense. The driver could drop the car back at Dublin Airport and Bus back to Belfast whilst the others take the sights (Cheap way?)or you pay the rental company to repatriate the thing (expensive way?)

Dunno about the Larne ferry -- not even sure if it runs, in March. Stena runs a ferry from Belfast harbor to Stranraer, though.

If Drop Off is too expensive, consider dropping the car off at Dublin and either taking the bus or train to Belfast. The Stranraer ferry is only 100 feet or so from the train station, with access to Glasgow and Edinburgh.
http://www.stenaline.ie/ferry/rail-and-sail/scotland/

Drive time, Cork to Dublin airport is about 3-3.5 hours, excluding stops.

Bob

I used the train from Cork to Dublin, Dublin to Belfast, and a Stena Line fast ferry to Scotland followed by a slow train to Glasgow. I don't remember the travel times but it's all available on line from public sources.

Found this on the PRONI site:

Distances and travel times (driving) to Belfast City centre:
From Distance (Miles) Travel Time
George Best Belfast City Airport 4 c.20 mins
Belfast International Airport 20 c. 40mins
Coleraine 56 1hr 15mins
Omagh 67 1hr 20mins
Londonderry 70 1hr 40mins
Enniskillen 82 1hr 45mins
Dublin 100 2hrs
Galway 195 4hrs 40mins
Rosslare 207 4hrs 30mins
Limerick 225 4hrs 50mins
Cork 260 5hrs 45mins

Bob

Thank you all so much for your feedbacks!
Anyone has suggestions on places/food/experiences that is a must-see?

We both love cultural experiences, a taste of local life and viewing some castles. Not that much on museums but maybe dinning and shopping at places locals would go. Was also thinking of going to the Guinness factory in Kilkenny, advisable?

Suggestions:

Blarney Castle just outside Cork is a don't miss. Actually it ain't much and climbing those stairs takes a sure foot but hey, you kissed the stone! There is a huge Waterford shop there also.

Guinness Storehouse in Dublin is a good tour, topped off by a pint on the house. The best chowda I ever had was at Ryan's FXB Grills in Dublin.

Just stay on the train in Glasgow and head for Edinburgh. Take a city bus out to Roslyn Chapel. It's a tiny old place undergoing repairs a few years back. The Witchery is an historic and fine restaurant, and at a price.

You can drive to London but there is a very nice train. I took a couple of nights in York on the way back to London. You definitely DO NOT want a car in London.

Bit tongue-in-cheek:
If you look for a place where locals love to shop, stop at the Kildare Outlet Shopping Village right on the M7 motorway when you drive North from Cork to Dublin

Great tips folks~! Will definitely work them into our schedule.
Appreciate all the advises so far.. keep em coming!

Is there any good offline maps we can download on iPhone/iPad?

Actually, if you do the Rail and Sail, you'll need to change stations in Glasgow, to take the train on, to Edinburgh. It's a short walk, through a pleasant part of town. We did it in 2007, hauling large rolling duffle bags, without any problem.

While it might be lunacy to have a car in London, I could make a similar argument against having one in Edinburgh, as well. Rosslyn is a short, inexpensive bus ride from city center and WELL worth a visit -- even if you are NOT a Dan Brown fan ...
Bob

Tlky-check out the "mapswithme" app. I have used it in Dublin ,and found it very useful to not have to carry a paper map.

Susan

Thank you njsusan! Will download the app!