Machine Cancels. 
The increase in volume of mail reached the point where additional manpower, no matter how cheap, was not the solution to processing mail. While tasks, such as sorting, would have to await computerization of the late 20th Century, the application of machines to simpler tasks, cancelling the stamps, for example, was within the reach of the 19th Century mechanical devices.
Four specific cancellers with unique cancelling patterns, can be found on Irish mail from about the turn of the century.
The Columbia machine, made by the Columbia Postal Supply Co, of New York, was first installed in Dublin in 1906 and later Belfast. It is identifiable by the 6 parallel lines and the rectangular city/date box. Here are many variations to this design. (Figure 1
The Krag machine, invented in Norway, is easily identified by the two rectangular city/date boxes between the killer lines. (Figure 2). The machines were installed in eight locations around 1914: Dublin, Cork, Limerick, Galway, Kilkenny, Monaghan, Londonderry and Ballymena. Only one slogan cancel is known “Feed the Guns” from Cork in 1918.
The Hey-Dolphin machine was also from New York. Originally designed in 1893, it underwent several modifications and was finally purchased by the British Post Office as the “Flier” model around 1915 for use in Dublin and Belfast. Its round city/date box is distinctive, and five killer bars were used. (Figure 3).
The Universal machine of Reneo Neopost was a later design, employing some of the better points of the Krag machine. These were initially installed in Cork, Limerick, Dublin and Belfast around 1920. Like the Hey-Dolphin, it had a round circle with the city/date and a trademark semicircular line under the date. These had seven wavy lines, except for Dublin which had 5. Both Dublin machines were destroyed in the Rotunda fire of 1922, but eventually replaced. (Figure 4).
The 1916 Uprising and subsequent guerilla warfare resulted in machines being moved, modified, or destroyed. Some of these machines had short life spans, while others continued on for over 50 years. See also Slogan Cancels.
Early Irish Stamp Cancelling Machines Until 1922, Schollmayer, M., FAI Volume 22, 2000.
Irish Postmarks Since 1840, Mackay, J, 1983.
Ireland – Transition, The Postal History of the Transitional Period 1922-1925, Dulin, C., MacDonnell-Whyte ltd, Dublin, 1992.
Collect British Postmarks, Whitney, J.T., British Postmark Society.