Upvote:1
An Irish biometric visa is marked ‘BC’ or ‘BC BIVS’ in the ‘Remarks’ section. If you have that then you do not need a visa to transit.
If you don't have that you will need a visa.
https://www.gov.uk/check-uk-visa/y/macedonia/transit/somewhere_else/no
https://www.gov.uk/check-uk-visa/y/macedonia/transit/somewhere_else/yes