The most recent calculation of the population of the island of Ireland is 6.4 million, with 4.6 million in the Republic and 1.8 million in Northern Ireland (stats here). Scotland's population is a wee bit less, at 5.3 million.
The largest 25 towns and cities across the whole island are (broadly speaking, using 2011 figures) listed below, with 12 of the 25 in Northern Ireland:
1 Dublin 1,045,769 2 Belfast 276,459 3 Cork 190,384 4 Limerick 90,757 5 L'Derry 83,699 6 Galway 72,729 7 Lisburn 71,465 8 N'abbey 62,056 9 Bangor 58,388 10 Craigavon 57,685 11 Castlereagh 54,990 12 Waterford 49,213 13 Drogheda 35,090 14 Dundalk 35,085 15 Swords 33,998 16 Bray 31,091 17 Ballymena 28,717 18 Newtownards 27,281 19 Newry 27,433 20 Carrickfergus 27,201 21 Navan 24,851 22 Ennis 24,253 23 Coleraine 24,089 24 Tralee 22,744 25 Kilkenny 22,179
(The bold ones are in Northern Ireland.)
So the total 'urban' population on the island is 2.6m people, with 40% of these in Dublin alone.
If you set aside the huge 'skew' of Dublin (as you might do with London in a GB context) the population of the rest of the cities and towns on the island of Ireland totals 1,531,837.
Of this, just over half (799463) are in Northern Ireland. And of the top 12 cities and towns, 7 are in Northern Ireland.
Visually speaking, it looks like this (circles representing size of population, not the physical size of the places).
However, island-wide, this means that around 3.8 million people, 60% of the island's total population, do NOT live in the biggest cities and towns. This is still a largely rural island. Over the years I've worked with Northern Ireland-based food companies seeking to expand into the Republic and to sell products there. A map like this is a fairly good indicator as to where the marketing efforts need to be targeted for maximum return.
But it's also a surprise to see how strong Northern Ireland's towns and cities are when compared with the rest of the island. Working harder in your own back yard is often a simpler task for SMEs than the risk and stretch of expanding to a new location farther from home.
1) 40% of the island's population is urban.
2) Just over half of the non-Dublin urban population is in Northern Ireland.
3) Greater Belfast and the towns around it total 577,840, equating to roughly 1/3 of the non-Dublin urban population on the whole island.
I'm sure a professional statistician will have more accurate figures and analysis than these.