You generally need planning permission if the change amounts to a 'material change of use' between different planning use classes, such as moving from residential to commercial use. If the new use falls within the same broad use class as the old one, or is covered by a specific exemption, permission may not be required, but this should be confirmed with the local authority.
Whether you need planning permission to change how a building is used depends on whether the change is 'material' in planning terms. The Planning and Development Act 2000 and its regulations organise uses into classes, for example residential, retail, offices, and light industrial, and moving from one class to a genuinely different one is treated as development requiring permission because it can change traffic patterns, parking demand, noise, and the character of the area.
By contrast, switching to a different activity within the same use class, or making an internal change that doesn't alter the building's fundamental planning use, may not need permission. There are also specific exemptions under Schedule 2 of the Planning and Development Regulations 2001 (as amended) covering some limited changes, such as certain small-scale home occupations, subject to conditions on scale and impact on neighbours.
Because 'material change of use' is a judgement call based on the specific facts, and getting it wrong can lead to enforcement action or difficulty selling the property later, it's worth either applying for planning permission if there's a reasonable chance it's needed, or seeking a Section 5 declaration from the local authority to get a formal ruling on the exemption question before proceeding.