A small residents-only gym will also be provided at ground floor level - but it cannot be used by the public without a separate planning application.
Plans to convert vacant commercial space in the centre of Naas into three new apartments have been approved.
The development covers 6 and 7 South Main Street and an adjoining building on Abbey Street.
The front building is a protected structure, and the permission includes conditions to preserve its historic character.
A small residents-only gym will also be provided at ground floor level - but it cannot be used by the public without a separate planning application.
Kildare County Council has attached twenty conditions to the project.
These include retaining certain internal features, specialist conservation methods for cleaning the exterior, and the installation of a plaque on the front of the building to acknowledge its historic significance in the town.
A decision letter states: "A small stone plaque shall be erected on the front of the building, and the plaque shall acknowledge the historic nature of the building as being the Post Office for Naas. The size, design, materials and text of the plaque shall be agreed in writing with the Planning Authority prior to commencement. The wording/dating may be sought from the Naas Local History Group."
Before construction begins, the developer must submit a full construction management plan, covering delivery routes, site access, noise controls and operating hours.
No work will be permitted on Sundays or public holidays.
The Council says the project is in line with town centre planning policy and will bring vacant space back into residential use.

Woman Hospitalised Following Single Vehicle Crash In Athy, Near Nicholastown
Kildare Among Top Counties For Animal Welfare Complaints – New Figures Show
Call For Maynooth University To Address Student Parking Overflow Impacting Nearby Estates
Drivers Still Dodging Penalty Points By Donating To "Poor Box"
Catherine Connolly Sworn In As 10th President Of Ireland
Father Warns Parents To Have Conversations With Their Children After Son’s Aerosol-Related Death
Kildare Childcare Places At Risk As Home Childminders Face Possible Commercial Rate Charges
Localised Flooding Causing Disruption In Kildare Town