Security – Watermarks. 
A watermark is a deliberate thin spot created when security paper is made. The purpose is to create unique paper that is not available to the public, and that is difficult to copy. It is created by passing the wet paper pulp of a screened roll (a “Dandy Roll”) that has the watermark made from bits of metal. This metal piece creates a slightly thinner spot in the paper while squeezing the water from the pulp. This appears on a stamp as a darker area when the stamp is placed on a dark surface and wet with watermark fluid. Another means of seeing watermarks is to use a mechanical device known as a Signoscope ®.
Ireland has used two different watermark designs since 1922 – the “S inside an E” format (Saorstat na h’Eireann) was used from 1922 to 1940 and the “E” format from 1940 to the early 1970’s. Since then, watermarks have not been used on postage stamps. Prior to 1922, watermarks of Great Britain were used on stamps sold and used in the British possession of Ireland. [All illustrations are as seen from the back of the stamp.]
Watermarks may be found normal, reversed, inverted, and reversed and inverted depending how the paper was inserted into the press. Other configurations are possible, such as sideways left or right. The SE watermark is known in almost all positions. First, in the normal mode, it can be inverted, or to the left, or right.