I'm no expert, but I'll try my hand at it – a word-for-word translation would be something like "It is good with the boys it that", or, roughly, "That is good with the boys", idiomatically "the boys like that". I believe "é" could be used as either "he" or a gender-neutral "it", while "sin" is a weird, not-quite-translatable particle(?) that follows the noun it is specifying and indicates "that"/"those"/etc. There's probably a more-technical explanation for that, but I hope that helped!
The key to understanding the difference between is maith leis na buachaillí sin and is maith leis na buachaillí é sin is recognising who/what is being liked - na buachaillí sin in the first case and é sin in the second.
Once you know what is being liked, you can figure out who is doing the liking - is maith (leis) is the first case and is maith (leis na buachaillí) in the second.
The difference would be that in the first example ("is maith leis na buachaillí sin"), "leis" would be "le + 3° person singular masculine", analogous to "is maith liom anraith sin" (I like that soup), while in the second example ("is maith leis na buachaillí é sin"), "leis" would be "le followed by an article", right? So it's just "unfortunate" that "le" looks the same when it's combined with different things. If this is right, then in the second example "é" is just there to separate "sin" from "na buachaillí", meaning that "sin" does not modify "buachaillí", right? Because it would be confusing due to Irish having adjectives and pronouns following the noun they modify ("boy that" instead of "that boy"). I hope I understood correctly since I'm trying to get to the bottom of the grammar as I advance in the course.
é is not "just there to separate "sin" from "na buachaillí". You can't use sin on it's own. To say "that X", you need a definite article - an X sin, Is maith liom anraith sin doesn't make any sense, it has to be an t-anraith sin. To say just "that", you need the é.
"I didn't know that" - ní raibh a fhios agam é sin
"Is that a problem?" - An fadhb é sin?
"did you get permission to do that?" - an bhfuair tú cead é sin a dhéanamh?
"say that in English" - abair é sin i mBéarla
The thing that is liked is the last item in the construction is maith le X Y.
In is maith leis na buachaillí sin, the last item is na buachaillí sin.
In is maith leis na buachaillí é sin, the last item is é sin.
That é is sometimes lost in speech, but grammatically, it's still there.
As Cait48 and galaxyrocker already explained in the earlier comments, le + an X is written leis an X and le + na Xanna is written leis na Xanna.
(I just used Xanna as a generic plural - there is no significance to it).
There are some unusual cases where le does not become leis before an, but you don't need to worry about them for now.