"Say that again. I have a little Gaelic."
Translation:Can sin a-rithist. Tha beagan Gàidhlig agam.
Although Dwelly's dictionary lists beagan as either a noun or an adjective, other dictionaries appear to list it only as a masculine noun, meaning a little, a small quantity, a few or a small number. I find it easiest to think of it that way, so beagan Gàidhlig can be thought of as a small amount of Gaelic, with Gàidhlig being in the genitive case, indicating of Gaelic. It's just fortunate that in the case of Gàidhlig, the noun does not change form in the genitive case.
beag, on the other hand, is an adjective, meaning little or small and, like most adjectives, follows the noun it describes.