The main railway walking available in County Donegal is on the former Londonderry & Lough Swilly Railway, which operated 99 miles of narrow gauge line from Londonderry to Tooban Junction, where separate branches led off to Carndonagh and Burtonport. A Google search for ‘Dismantled railway Letterkenny to Burtonport’ will bring up Ireland’s travel guide (Active Me), which shows the whole line on a zoomable map.
The eastern end of the L&LSR’s Burtonport branch, which, despite all the odds (such as never making a profit), survived until 1940 for passengers, and 1947 for freight. The Burtonport-Meenbannad section is now promoted as the Burtonport Old Railway Walk, which came about when the trackbed out of Burtonport was cleared in 2009 to repair a water main that had been damaged by heavy snow. The cleared section was gradually developed, and extended, as a very scenic walkway, with help from the local community. Parts of the route have a sealed surface, but in places the old line is narrow and exposed, eg. on high embankments, which are best avoided in stormy weather.
The six miles from Falcarragh to Creeslough is another section of the Londonderry & Lough Swilly Railway, traversing the rocky terrain to the north of Muckish Mountain. Note that there are stepping stones over the Agher River. The main photographs on the websites for the Burtonport section make it clear that it is easy going, with a sealed surface for most of the way, through stunning scenery. You could get a wheelchair/pushchair along there easily enough, but there are places where two wheelchairs travelling in opposite directions would have to do a spot of jousting (Jeff Vinter)