Renovation costs vary more widely than new-build costs because they depend on the condition of the existing house and how much of it is being replaced. A cosmetic refresh, a services and energy upgrade, and a full strip-back refurbishment are three very different budgets, and only a survey of the actual building can tell you which one you are facing.
Renovation budgets are dominated by unknowns: the condition of the roof, wiring, plumbing, windows and structure. A pre-purchase or pre-works survey by a chartered building surveyor or engineer is the single most useful step in setting a realistic figure, because it converts unknowns into priced items.
Energy upgrades often form a large part of a renovation. SEAI administers grant schemes for insulation, heating and other measures, and the scope and eligibility of these change over time, so current details should be checked directly with SEAI before relying on them in a budget.