Looking back on our Irish Geography classes so long ago, there were two unalterable facts drummed into us!
1. Highest Mountain in Ireland: - Carrantouhill (3,414ft high) in the Macgillacuddy Reeks, Co Kerry
2. Longest River in Ireland & Britain:- The Shannon - 240 miles long (380km), rising in the Cuilcagh Mountains in Co Cavan.
Can you imagine if a peak in another part of the Macgillacuddy Reeks was re-measured and discovered to be 3,614 ft in height! 200 ft taller than Carrantouhill !! There would be great shock, uproar and excitement over this discovery.
Or, imagine if it was discovered that there was a river 260 miles long (410 km) flowing through Ireland, 20 miles longer than the Shannon River!! – Impossible!!
I’m afraid there is no undiscovered 3,614 ft giant mountain lurking about!
However, there is a river flowing through Ireland whose length, from source to mouth is approximately 260 miles (410 kms)!
Of course this does involve the River Shannon.
Yes, the Shannon River rises at the Shannon Pot near the Cuilcagh Mountains in County Cavan. It then flows west for a few miles, then south into Lough Allen in Co Leitrim. It leaves Lough Allen and continues south to be joined by the Boyle River about a mile above Carrick-on-Shannon.
It is later joined by other tributaries such as The Inny, The Suck, The Brosna and The Fergus on it’s journey to the Shannon Estuary.
Now, the source of the Boyle River is to be found on the Mayo/Roscommon county boundary, a few miles north of Ballyhaunis. The source river is called the Annaderryboy River. This river is fed by the outflows from Lough Roe, Lough Nanoge and Lough Urlaur (Co Mayo), and Lough Errit (Co Roscommon). The Annaderryboy River is joined by the Black River and flows through Cloonagh Lough, before becoming the Lung River. The Lung River then flows for several miles, passing close to Ballaghaderreen, before joining Lough Gara which is also fed by the Breedoge River. After Lough Gara the water flow is termed the Boyle River which flows past the town of Boyle into Lough Key and onwards to meet the Shannon River, just above Carrick-on-Shannon.
Now, even to my untrained eye, on looking at a map, this tributary (Annaderryboy/Lung/Boyle to Carrick-on-Shannon) always appeared to be appreciably longer than the Shannon River proper (Shannon Pot to Carrick-on-Shannon)
Proving this point has always been frustratingly difficult! – Resorting to using dividers on the map or, worse still, a length of thread, wasn’t entirely satisfactory.
Until I recently discovered two methods of achieving extremely precise measurements.
I found a brilliant inter-active groundwater web-mapping map of Ireland on the Geological Survey of Ireland website.
Groundwater public viewer site:- http://spatial.dcenr.gov.ie/imf/imf.jsp?site=Groundwater
There is a tool for zooming in very close (less than 100 meters to 1 inch) and there is a tool for measuring and accumulating distances. Combined with a Pan tool for moving the map, this produces an extremely accurate method for measuring lengths of streams and rivers!
Google Maps also has an “easy to use” Distance Calculator measuring tool. Very effective and speedy, when the map resolution is good!
The use of detailed Ordnance Survey Map Discovery Series maps No’s: 26,32 & 33 (available in Eason) is also a great help in keeping track of where you’re at.
I decided to start at Cloonroe Bridge, on the Annaderryboy River (just after the meeting of the waters of Lough Urlaur and Lough Errit) as a starting point, as I had recently visited the location. (I needed to ensure that the line on the map wasn’t some dried up gully - I was pleasantly surprised!)
This is the result of my measurements using G.S.I. Web-Mapping:-
Here I found that the distance from Cloonroe Bridge (on the Annaderryboy River) to the Shannon (just above Carrick-on-Shannon) is 50.46 miles (80.6 km).
The distance from the furthest headwater of the Shannon (Owenmore River) in Co Cavan to the same point (just above Carrick-on-Shannon) is 35.01 miles (56.34 km).
Using the Google Maps Distance Calculator:- Cloonroe Bridge to Carrick-on-Shannon is 50.17 mls (80.7 kms) with Shannon (Owenmore River) to Carrick-on-Shannon is 33.88 mls (54.52 kms).
Though the results of these measurements are not exactly the same, there can be no doubt which source is the longer!
Both measurements make the tributary (Annaderryboy/Lung/Boyle) from the Cloonroe Bridge to Carrick-on-Shannon 15+ miles longer (20 miles longer if you include Urlaur Lough & Lough Roe).
Thus, this source, the Annaderryboy River, is the most distant natural source in the Shannon system.
The source coming from Lough Errit is also of similar length.
On the strength of these measurements, I can state with absolute confidence that the Longest River in Ireland is the Annaderryboy/Lung/Boyle/Shannon River, sourced in eastern Co Mayo/Co Roscommon. - NOT the Shannon River, from the Shannon source in Co. Cavan.
I am surprised that this information hasn’t been publicised before now!
These facts can be verified by anybody with a PC and the Internet.
It’s as if the geographers, mappers and data collectors are happy that the longest river in (dare I say it!) “The British Isles”, rises in the Shannon Pot and that’s the end of it!
Are the waters that come out of Connaught, of a lesser vintage? More dilute?
Standing at Urlaur Abbey, overlooking the picturesque Urlaur Lake (complete with swans!), near Kilkelly in Co Mayo, it’s amazing to think that it’s waters flow further than any other waters in these isles! Wouldn’t it be fitting if this fact was recognized!
Shouldn’t the fact, (that Ireland’s longest river is over 400 km long and not just 380 km long), not matter?
Just supposing Carrantouhill was always thought to be 3,200 ft in height, (slightly lower than Scafell Pike - England’s highest peak), and on remeasurment, was found to be 3,414 ft tall - wouldn’t we be chuffed!
So, what’s the harm in our longest river being longer?
Size does matter! Length does matter!
Isn’t it about time our data was updated?
Now which river is the 2nd longest in Ireland? The Suir? The Blackwater? Or possibly the Barrow? . . . . . . . hmmmmmmmm!!
References (From Wikipedia):-
1. There is no universally agreed upon definition for determining a stream’s source. The United States Geological Survey (USGS) states that a river’s “length may be considered to be the distance from the mouth to the most distant headwater source (irrespective of stream name), or from the mouth to the headwaters of the stream commonly identified as the source stream”. As an example of the second definition above, the USGS at times considers the Missouri River as a tributary of the Mississippi River. But it also follows the first definition above (along with virtually all other geographic authorities and publications) in using the combined Missouri - lower Mississippi length figure in lists of river lengths around the world. 
2. For the purpose of determining maximum length a river’s “true source” is considered to be the source of whichever tributary is farthest from the mouth. This tributary may or may not have the same name as the main stem river. For example, the source of the Mississippi River is normally said to be Lake Itasca in the U.S. state of Minnesota, but the most distant source in the Mississippi system is that of the Jefferson River in the state of Montana, a tributary of the Missouri River which in turn is a tributary of the Mississippi. When the Mississippi is measured from mouth to this farthest source, it is called the Mississippi-Missouri-Jefferson. Furthermore, it is sometimes hard to state exactly where a river begins, especially rivers that are formed by ephemeral streams, swamps, or changing lakes. In this article, length means the length of the longest continuous river channel in a given river system, regardless of name.
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