I think the total passenger numbers within Northern Ireland is running at about 11 million a year, but I don't have a breakdown by route.
If you read the history of railways in Northern Ireland in the 1950s and 1960s, they were seen as anachronistic and uneconomic. The car and the bus was the way forward and repeated Unionist Governments closed as much down as they could, including plenty of lines in what might be perceived as Unionist territory eg the Belfast to Newcastle line. And there were several attempts to close everything else down, save perhaps for the Belfast Dublin line. I dont think sectarianism came into it, just economics and a bias against rail as a good form of transport. Fortunately thats changed a bit in recent years.
You say that the service to Londonderry/Derry is not very good. Its a single track line all the way from Londonderry to Mossley West on the outskirts of Belfast with 10 departures form Waterside station on weekdays. Thats twice the service frequency on offer 30 years ago, and the main reason there isnt a greater service from Londonderry is lack of passing places (theres none between Londonderry and Castlerock), something about to be fixed by the Bellarena loop when it opens later this year. No timetable has been published yet but the service is bound to be increased significantly, possibly to an hourly frequency, since otherwise putting in the passing loop would be a gross waste of money.
This is from a news release recording what the NI Transport minister said in 2014 (I cant get the link to work):
Passenger numbers on the Coleraine to Belfast railway increased by a third between 2011/12 and 2013/14 equating to an increase of over 400,000 passengers journeys in three years, taking passenger numbers from over 1.2million in 2011/12 to over 1.6million in 2013/14.
The Minister said: There has also been passenger growth on the Londonderry to Coleraine stretch of the railway network, and the completion of Phase 1 of the track relay on this stretch has played a key role in this.
The introduction of a new rail timetable targeted at the Coleraine to Belfast service, introduction of new trains and continued investment in infrastructure has clearly helped to generate this growth. It is important that we continue to invest in our rail infrastructure and ensure rail fares are competitive.
Across Northern Ireland rail passenger journeys have increased by approximately 2.5 million over the period from 2011/12 to 2013/14. The Minister welcomed the significant upturn in rail passenger growth. He said: In recent years there has been a significant growth in numbers choosing to use the railway as a mode of transport across Northern Ireland. I believe rail travel offers a safe, comfortable and efficient option, particularly for commuters and inter-city passengers.
The Railway Investment Prioritisation Strategy published recently sets out the priorities for my departments investment in railways capital grant over the next 20 years. This includes proposals for upgrading the Coleraine to Belfast corridor.
My first priority is track maintenance and the improvement of passenger capacity on the existing network. The strategy includes a number of projects on the Northern line between Coleraine and Belfast for track works, station improvements and Park and Ride facilities.
As passenger demand increases, Translink plans to purchase up to 60 additional carriages which will allow the introduction of longer trains or additional services to extend passenger capacities, including on the Northern line.
The capacity of the Northern line to carry more trains will depend upon removing potential track bottlenecks around Belfast. There are proposals for dualling of the single track crossing of the Lagan at Dargan Bridge and adding an additional line from Lagan junction to Central Station. This will help improved timetabling of services into Belfast.
In the longer term, the Strategy commits Translink to consider options to enhance timetabling and shorten journey times on the Northern line by either dualling the existing single line track between Bleach Green and Antrim or re-opening the Antrim to Knockmore/Lisburn line.