The registration marks on all gliders must conform to the regulations. The marks must consist of IE-XXX or EJ-XXX and are allocated by the IAA. They must be of a colour that contrasts with the background. The conditions and specification are contained below.
Extract from SI 107 of 2015 Nationality and Registration of Aircraft
(i) Wings. The height of the marks on the wings of heavier-than-air
aircraft shall be at least 50 centimetres.
(ii) Fuselage (or equivalent structure), vertical tail surfaces or other
structure. The marks on the fuselage (or equivalent structure) and
on the vertical tail surfaces or other structure of heavier-than-air
aircraft shall be at least 30 centimetres.
(iii) Rotorcraft and special cases. If a heavier-than-air aircraft does
not possess parts corresponding to those in paragraphs 3(c)(i) and
(ii), or if the parts are too small to accommodate the marks
described therein, the measurements of the marks shall be such
that the aircraft can be identified readily. [IGSA Note: This also applies to gliders]
4. The type of characters for nationality and registration marks shall be as
(a) The letters shall be capital letters in Roman characters without ornamentation.
(b) The width of each character (except the letter “I”) and the length of
hyphens shall be two-thirds of the height of a character.
(c) The characters and hyphens shall be formed by solid lines and shall
be of a colour contrasting clearly with the background. The thickness
of the lines shall be one- sixth of the height of a character.
(d) Each character shall be separated from that which it immediately precedes
or follows by a space of not less than one-quarter of a character
width. A hyphen shall be regarded as a character for this purpose.