"Feumaidh mi falbh."
Translation:I need to go.
falbh is not a verb, it is a masculine noun and means going away; (the act of) leaving, the sentence literally translates to I need leaving (but of course that’s not idiomatic English, so it better translates to I need to go with an infinitive, but Gaelic doesn’t have infinitives). Such nouns naming actions are called verbal nouns, they are often formed from verbs and often translate to verbs in other languages, but they are grammatically nouns and act like nouns.
a’ falbh is a prepositional phrase meaning at leaving, you use it in sentences like tha mi a’ falbh I am leaving, literally I am at leaving.
But if you want or need leaving, you don’t use this at thing, so you just say that you need falbh, not