am is time. the t- is there because it's a masculine noun beginning with a vowel following the definite article.
Orthography and convention. Basically, you use a hypen between it if lowercase, and none if uppercase.
An t-am, but an tAontas Eoprach
The pronunciation was difficult for me. It sounded like "am" was pronounced "ow-n".
I've since come across an explanation about the pronunciation. This one seems consistent with Munster pronunciation, i.e., that before "nn" and "m" the "a" gets a pronunciation more like "ow." At least that's according to the Teach Yourself Irish book from 1961 that I downloaded from Internet Archive.