"I have shoes and underpants on."
Translation:Tha brògan agus drathais orm.
It's neither. "Tha" is the verb and means "is" or "are".
In sentences of this type the I and the you are included in the last word there, which in this case is "orm". It is a contraction of "air mi" which means "on me", and the literal translation of the sentence is "There are shoes and underpants on me", which is the way Gaelic says "I am wearing shoes and underpants."
If the desired sentence was that you were wearing shoes and underpants, these garments would have to be "at you" rather than "at me", which needs the contraction of "aig thu" which is "ort". So, "Tha brògan agus drathais orm" means I have shoes and underpants on while "Tha brògan agus drathais ort" means you have shoes and underpants on.
I don't think you have read the Tips and Notes which you really need to, or you'll never get this. This specific point is covered in the notes attached to the "Clothes" skill, but I'm getting the impression you haven't read any of them. The entire collection is together on one page here for your convenience. https://duome.eu/tips/en/gd