Cork radius map: 5, 10, 25 and 50 kilometre circles

If you want one landmark to calibrate against, use Douglas. It’s 3.6 km south-east of the centre, so it sits almost exactly on the inner circle. Everything in between scales from those two.

One caution before you lean on the table. A GeoNames centre is a single point, usually somewhere near the historic core, and a city isn’t a point. Douglas is listed at 3.6 km because that’s the gap between two reference points. Its nearer edge is closer than that, probably by a good margin.

Place names, coordinates and populations come from GeoNames, which the site uses under CC BY 4.0.

Area climbs with the square of the radius, and it catches people out every time. The 5 km disc around Cork covers 79 km². The 25 km one covers 1,963 km², twenty-five times as much for five times the distance. By 50 km you’re looking at 7,854 km² of Munster and whatever lies past its edges.

Those areas are worth holding against the ring counts. 7,854 km² inside the outer circle, and GeoNames puts 3 named settlements in all of it. A density figure off that would be meaningless, so there isn’t one here. What the areas are good for is sizing: knowing the 25 km ring covers 1,963 km² tells you how much ground a service area at that radius has to reach across.

Straight line, not drive time, and the difference bites hardest on the big ring. Dublin comes out at 219 km from here as the crow flies, and nobody has ever driven it in a straight line. Use the circle as an outer bound. Treat anything sitting near the edge as optimistic, because real travel distance beats the radius every time and the gap grows with whatever the terrain throws in.

Plenty of sites draw a plain circle, label it drive time and leave it there. Those are worth ignoring. A proper isochrone needs a road graph, turn restrictions and some idea of traffic, and it changes shape by the hour. What a circle gives you instead is a bound that doesn’t move: nothing outside the 25 km ring is within 25 kilometres of Cork, whatever the roads do.

On the arithmetic. Distances are great-circle distances on a sphere of radius 6,371.0088 km, worked out with the haversine formula, and each bearing is the initial heading you’d take leaving Cork. Areas use the spherical cap rather than πr², which shaves a hair off the flat answer. About one part in seventy thousand at 50 kilometres. Invisible, and true anyway.

One oddity you’ll notice if you compare pages. Population counts come from GeoNames, which records what the national statistics office last published, so some are a few years old and a couple are more. They’re good for ranking and for rough sizing. The figure carried for Cork is 224,004, which for some cities means the municipality and for others the whole urban area, so compare it with care.

What people actually use these rings for, roughly in order of how often it comes up: delivery and service areas, finding somewhere to live within a sensible distance of work, sizing a catchment for a shop or a clinic, checking how far an event’s noise and traffic will carry. Straight-line distance is the right first pass for all of them.