OK, I'll start with what you already know.
leat is the prepositional pronoun "with you" and as you also know, toil means "pleasure".
Am bu is the only bit you likely wouldn't know. "Bu" is a form of the word "biodh" meaning "would". As you know, every verb has a question form. For example "tha" as a question is "A bheil". The question form of "biodh" is "am bu"?
So the whole phrase means something along the lines of "would pleasure be with you"
"Bu" is a form of the word "biodh" meaning "would"
bu is the conditional of is. It's the same tense (mood, strictly) as bhiodh (NB: bhiodh), but really a different verb (is v bi).
The question form of "biodh" is "am bu"?
The question form of bhiodh is am biodh?. The question form of bu is am bu?