There is no single fixed distance that applies everywhere; how close you can build to a boundary depends on whether the works qualify as exempted development, Building Regulations fire safety separation rules, and your local authority's development plan policies. Extensions and outbuildings have specific exemption limits, but anything beyond those, or affecting the neighbour materially, generally needs planning permission.
How close you can build to a boundary in Ireland is governed by a combination of rules rather than one simple distance rule. The Planning and Development Regulations 2001 (as amended), Schedule 2, set out exemptions for certain extensions, garden structures and boundary walls, some of which include specific conditions relating to proximity to a boundary, height, and floor area, but these limits vary by structure type and by whether the property is a house or other use.
Separately, Building Regulations Technical Guidance Document B (Fire Safety) requires certain wall constructions, restrictions on openings such as windows, and limits on roof coverings where a building is close to a boundary, to reduce the risk of fire spreading to a neighbouring property. These requirements apply regardless of whether the works are exempted from planning permission.
Where a proposal falls outside the exemptions, for example because it exceeds the permitted floor area or height, or is a new dwelling rather than an extension, full planning permission is required, and the planning authority will assess the proposal against its development plan, taking into account overshadowing, privacy, and the character of the area, in addition to the exact boundary distance.