"I have a white shirt."
Translation:Tha lèine gheal agam.
What makes a noun masculine or feminine is.... not really easy to tell. It's mostly stuff you just have to learn with the words. And it's not always obvious from what the word is. For example the word boireannach (woman) is masculine, lèine -shirt, is feminine, but blobhsa (blouse) is masculine. One of the best ways of learning is to put it in your memory with a lenitable adjective like this, so you remember which way it goes.
adjectives undergo "lenition" after feminine nouns. It's rather complicated...more details here: https://gaelicgrammar.org/~gaelic/mediawiki/index.php/Lenition