You learned very early in the course that bean becomes an bhean because feminine nouns are lenited after the singular definite article an in the nominative case. But that's not the full story. You can only lenite a consonant. For words that start with a vowel, it is masculine nouns that are marked after the singular definite article, and they are marked with a t- prefix.
aláram is a masculine noun, so you get an t-aláram.
Interesting, my parents from the Gaeltacht used to refer to the Halaram clock in English. I guess if they said "the alaram" clock they would have a vowel ending next to a vowel beginning and it wouldn't flow so well. Just like we pronounce "thee alarm clock", and not "thuh alarm clock"