Tuesday 13 May 1845
[…] afterwards we met P McHenry and we went to the Atmospheric at Dalkey we travelled at about 30 Miles Pr Hour it is the most Zig Zag narrow disgraceful thing I ever beheld
David Brooke ed The diary of William Mackenzie, the first international railway contractor Thomas Telford Publishing, London 2000
Irish waterways history 
There is a reason for it being “the most Zig Zag narrow disgraceful thing”. The normal procedure for building a railway line was to get an Act of Parliament which allowed the builders of a railway to do many things, including compulsorily purchase the relevant land and also cross the King’s Highway on the level.
The Dublin and Kingstown Railway (owners of the Atmospheric Railway to Dalkey) didn’t do this. Instead, they did a deal with the Treasury to lease one line of the Dalkey Quarry Tramway on which to build the railway. Absent an Act of Parliament, they couldn’t alter the route of the line (hence it followed the curves of the quarry tramway) nor could they cross any roads on the level. This meant that they had to build a cutting to carry the Atmospheric Railway under the relevant roads (it is interesting to note that you needed permission to cross the King’s Highway on the level but could gouge a 2 mile long cutting through the countryside without so much as a by your leave from anyone!).
To this day, this cheapskate decision by the D&KR has had ramifications on the railway operator of the line from Dun Laoghaire to Dalkey, limited as it is by sharp curves and a propensity to flood.
Sorry about the delay: blame WordPress! bjg