Garage conversions are exempted development, within limits.
The proposed development constitutes a purpose built stand-alone bedroom.
It will need permission.
Proceeding on the basis that, size-wise, you are okay is a nonsense.
You are considering undertaking unauthorised development and under the Planning and Development Act 2000, you are wide open.
A narky neighbour could (i) refer you to the planning enforcement section of your local authority or (ii) take a section 160 action against you in court.
Bear in mind that extending out the back of your house, within limits, is exempted development.
I think you are setting yourself up for a fall, if the standards you build to do not comply with the regulations as Sconhome has confirmed.
Despite looking straightforward to build log cabins are anything but simple to build correctly.
Log Cabins are commonly used in colder climates, where despite the weather being more severe, the temperature and humidity ranges for a given season usually fall within defined limits.
Often they are erected in heavily wooded areas, well sheltered from wind and driving rain.
None of those conditions arise on a typical Irish site, the wildly varying Irish weather can open up even seasoned timber and unless the logs are prime timber, I anticipate they will tend to warp and open their construction joints.
You need to get supplied by an experienced log cabin builder in this country - are there any?
If not you are importing technology which may be unsuitable.
If you are using the insulation look-alike log cabin construction, you may find the room becomes unbearably hot in summer.
Do your research first, and be sure to check the logs as they arrive if you proceed with this.
Unprocessed timbers carry their parasites with them, and no one will thank you for importing some new tree bug or disease from Scandinavia or Canada.
FWIW
ONQ.