Railfanning in the British isles

The British Isles (Republic of Ireland, Northern Ireland, Wales, England, Scotland, Isle of Man, and Isle of Wight) have a ton of historical rail operations, and rail museums. There’s also a vigorous heavy rail system that can take several weeks to tour entirely. The Cornwall Riviera and the Caledonian trains provide overnight service from London to Cornwall and Scotland.

Passes (are they useful or not):

Britrail passes are expensive, but the cost of last minute tickets are expensive too. If you want the convenience of being able to get on the next available train, and you are planning 3-4 long distance train rides in England or Wales over the period of a few days, you can save money with a pass. (Scotland and Northern Ireland have cheap trains, so passes are more expensive. Ireland has unusually high pass prices—you’re usually better off buying a ticket.)

Buying rail and bus tickets:

England, Scotland and Wales.
The absolute cheapest way to get around the UK is by Trainline or Loco2, but you’ll need to book a week ahead to save money. Load the app on your smartphone, or use the web interface. https://www.thetrainline.com You can pay by credit card or paypal, and the ticket can either be delivered to your phone, or picked up from one of the thousand of automated ticket machines at UK train stations. As of 2018, the Trainline is phasing in a service fee (less than £3 per purchase). Many rail fans are switching over to Loco2, or purchasing directly from the Network Rail website to avoid the fee.
Northern Ireland:
NI has their own ticket selling app and website: http://www.translink.co.uk
Belfast Visitor Pass: https://visitbelfast.com/pages/belfast-visitor-pass
You’ll need to purchase this from the Belfast Tourism office, either by mail or in person. Available for 1-3 day intervals, cost’s £5-6 a day, provides free Belfast-metro area bus and train transportation, and discounted entry to Cultra, and the Titanic Experience.
Special trip note— there are many commercial tour bus companies that offer rides to Giant’s Causeway. You can take NI Rail from Belfast too: take the NI Rail train to Coleraine (90 minute trip). From there change to the 172 or the 402 bus to Giant’s Causeway (30 minute trip). You can work in a trip to Bushmills Distillery too—it’s along the bus route. Just get off, and catch a later bus.
Republic of Ireland:
Train Tickets— best deals are available by purchasing through the Irish Rail website. http://www.irishrail.ie/
Bus tickets— best deals are available by purchasing through the bus company website. http://www.buseireann.ie/ (the main bus company). https://www.aircoach.ie/ (regional buses to the airport). http://gobe.ie/ (discount regional bus co).
Leap Card— Works on Dublin, Cork City and Limerick City-area local trains, buses and light rail.  (including the bus from Cork Kent station to Cork Airport) https://www.leapcard.ie/Home/index.html
This is a declining balance card, that you can top up using a credit card at ticket machines (a rail and light rail stations). This is the only way to purchase a daily pass that works on Dart, Luas, buses (Including the Dublin Airport 747 bus), and commuter rail in Dublin—the system features a €10 daily, €40 weekly fee cap. You can buy a Leap Visitor card (€10 for daily, €40 for 7-day) at Dublin Airport (Terminal 1: at the bus information counter. Terminal 2: at the Spar convenience shop in the arrivals area). Regular Leap cards can be purchased from hundreds of convenience stores around Dublin, Cork Waterford and Limerick They cost €5, but supposedly this is refundable, all cards come with a €5 credit pre-installed.

Some points of interest:

Online map of all modern and historic rail lines in UK and Ireland: https://www.railmaponline.com/UKIEMap.php

Calendar of upcoming UK railfan trips:
https://www.railadvent.co.uk/

Overnight trains:
Night Riviera (Cornwall to London train) and the Caledonian (nightly train between London and five cities in Scotland).
Seat 61’s exhaustive and frequently updated guides (including how to buy cheap tickets).
Riviera: https://www.seat61.com/Cornwall-sleeper.htm
Caledonian: https://www.seat61.com/CaledonianSleepers.htm
The Caledonian upgraded their trains with new sleepers in May 2019. Besides bunk-style rooms, there are double rooms featuring double bed, and large first-class rooms.
Mainline historical train operators:
Midland Pullman https://midlandpullman.com/ New 2021 startup, uses an upgraded repurposed HST trainset to run day trips around the UK.
West Coast Railway https://www.westcoastrailways.co.uk/ — Runs the Harry Potter Train, and a full host of excursions. This company owns a fleet of historic equipment, including steam locomotives that are qualified to run on the mainlines in the UK.
Scottish Rail Preservation Society http://www.srps.org.uk/ — Based in Bo’ness, Scotland (commuter suburb of Edinburgh), they offer frequent weekend excursion trips over the summer months using their historic coaches and engines. Their museum is worth a visit, but they don’t usually have equipment running. Meg and I have traveled with them a few times— they’re a volunteer group. Really nice folks, and their equipment is in great shape and very comfortable to ride in.
UK Railtours https://www.ukrailtours.com/shop/ — This operation runs about a dozen daytime trips around the UK every year.
National Railway Museum in York https://www.railwaymuseum.org.uk/whats-on — This is the the preeminent railway museum in the UK. Going to England and not visiting York, is like going to Flagstaff and not seeing the Grand Canyon. It’s mostly a museum, but there is usually operating equipment on the weekends.
Belmond https://www.belmond.com/trains — Belmond runs two high-end, high-price train operations serving the UK. Their services are seasonal (trains run May through October. The Scottish train: the Royal Scotsman, and the British Pullman typically offers day-train excursions including exquisite dinners, departing from London. Sadly, their Irish train, the Hibernian, has been discontinued.
Brighton Belle http://brightonbelle.com/ — Not in operation right now. (Train set is undergoing an extensive, multi-year rebuilding project.)
Railway Preservation Society of Ireland https://www.steamtrainsireland.com/ — Runs a seasonal schedule of excursion trains around the Republic of Ireland and Northern Ireland. Owns mainline-qualified steam locomotives and coaches.Their museum in Whitehead, Northern Ireland runs steam locomotives on the weekend, and they have monthly meetings.
Railtours Ireland https://railtoursireland.com/ — Using coaches provided by the Railway Preservation Society of Ireland, and locomotives from Irish Rail, this offers day trips, and multi-day trips around the Republic of Ireland.
Tailte Tours https://www.tailtetours.com/ is a new (2022) Irish excursion train operator. Typically their trips use RPSI or Irish Rail equipment, or and feature unusual destinations and routes.
Museums:
Get the Guide book—The Steam Heritage Railway Guide (https://www.steamheritage.co.uk/).  Published annually, and usually released around May 1, this directory lists hundreds of small railroad museums–most of which are weekend operators. Typically they have special events on two or three weekends over the summer. The guide features a handy calendar that lists events at the various railroads by weekend. If buying from the USA, it’s also available from Amazon.com.
Rail and Transportation Museums in Ireland:
The National Transportation Museum in Ireland — Howth (Dublin Area—Ride the Dart to Howth, it’s a 1-2 mile walk from the station) http://nationaltransportmuseum.org/
This place is seldom open, even during announced hours. Contact them before going.
Rail Preservation Society of Ireland Museum — Whitehead (Belfast area— ride the commuter rail there) https://www.steamtrainsireland.com/museum-tickets
Relatively new museum. Meg and I visited them before they opened. They have a lot of nice stuff including working steam engines, but they are small.
Ulster Folk and Transport Museum — Cultra (Belfast area— ride the commuter rail there) https://www.visitardsandnorthdown.com/things-to-do/ulster-folk-and-transport-museum Great museum!
West Clare Railway — Moyasta (West of Limerick in Clare’s remote coastal area, you’ll need a car http://www.westclarerailway.ie/
This is a very small operation, but there’s a lot of interesting equipment at the site. This is a 3-foot gauge line (Ireland’s extensive narrow gauge operations were all abandoned in the 1950s, though 3-foot gauge railways are used in the commercial peat bog operations)
Also:
Listowel and Ballybunion Railway. https://en.wikipedia.org/wiki/Lartigue_Monorail#Listowel_and_Ballybunion_Railway
It’s a recreation of a historic monorail system running on a short section of the original line.
Waterford and Suir Valley Railway. https://en.wikipedia.org/wiki/Waterford_%26_Suir_Valley_Railway
This is a new narrow gauge line that was laid over a former Irish gauge line. Line runs alongside a section of a long rail-trail that runs between Waterford City and Dungarvin