Planning and Development Regulations, 2001
specifies the limits for auctioneers' signs to be exempted development.
Schedule 2, Part 2, Class 9
Advertisements relating to the sale or letting of any structure or other land (not being an advertisement structure) on which they are exhibited.
1. The area of any such advertisement shall not exceedâ€”
(a) in the case of an advertisement relating to the sale or letting of a house, 0.6 square metres,
(b) in the case of an advertisement relating to the sale or letting of any other structure or land, 1.2 square metres.
2. Not more than one such advertisement shall be exhibited on the structure or other land.
3. No such advertisement shall be exhibited, and no advertisement structure erected for the purpose of exhibiting such advertisement shall remain on the structure or land, for more than 7 days after the sale or letting to which the advertisement relates.
If your neighbour is selling a site or a house plus site then maybe they can use the larger size.