Those who walk the Dublin Hills will be familiar with the name Seahan as published by the Ordnance Survey and it’s burial cairn etc. When I first started looking more closely at placenames in these hills, I soon realised this was an error. There are several sources that point to the name Seechon and variants e.g. in Malachi Horan’s account and Weston St.John Joyce’s writings. And of course, it’s still known locally in that form.
It’s only recently though that I spotted the probable origin of the form Seahan. If we go back to 1837, Eugene Curry was charged with investigating the names and antiquities of the area. To his credit he did make efforts in the Glenasmole area to collect information from local people. Writing his report back, he describes Suíghchán as a high mountain near Ballynascorney with cairn & trig station and later again names it as Seechaun Mountain. So how did ‘Seahan’ end up on the final maps? For whatever reason, Curry’s fieldwork was ignored. In fact it appears that John O’Donovan had submitted all the map names prior to Curry’s fieldwork.
The answer I think lies in the ‘County Map’ of the time. Local government was to the fore in these times and many if not all counties had commissioned maps of their jurisdictions in the late 1700s and early 1800s. And the Ordnance Survey gathered placenames from these, they usually appear listed as the ‘county map’ in the OS name books of sources. Dublin County had a number of county maps, however the current map in 1837 appears to have been Duncan’s Map of County Dublin of 1821. There is good indication of this as many of the names on same can be seen transcribed as is onto the OS Six Inch maps.
However Duncan’s map is a little hard to read in places – you’d want to be examining it closely. Have a look at the extract here, what’s written is ‘Sechen Mount’ which is compatible with Seechon/ Seechaun etc. However the ‘c’ could easily be taken as an ‘e’, so O’Donovan likely read Seehen, didn’t like that form and changed it to Seahan.