Well picked up! Taken on its own, that phrase is ambiguous. You cannot write "le na buachaillí" as a translation of "with the boys", because le becomes leis when written before an or na. It would almost be possible to alternatively interpret the sentence as [Is maith] [leis na buachaillí] [sin] and tor translate it as "The boys like that" - but that can't be correct, because the word sin never appears on its own like that. The correct way to say "The boys like that" would be Is maith leis na buachaillí é sin.
De Bhaldraithe includes a couple of examples where le na does not become leis na, in particular where the le is part of a phrase like is maith le:
"To do sth. for filthy lucre" - rud a dhéanamh ar mhaithe le na pinginí http://www.teanglann.ie/ga/eid/lucre
"Don't wash your dirty linen in public" - ná lig do náire le na comharsana http://www.teanglann.ie/ga/eid/linen
"He threw in his lot with the rebels" - chuaigh sé i bpáirtíocht le na reibiliúnaigh http://www.teanglann.ie/ga/eid/reibili%C3%BAnaigh
"It is a long time since I saw him" - ní fhaca mé le na cianta é http://www.teanglann.ie/ga/eid/long
"Watering of the eyes" - uisce le na súile http://www.teanglann.ie/ga/eid/watering
I don't find any similar examples in the NEID, or in Ó Dónaill, so I don't know if this suggests that these are dialect variations that have fallen out of favour, but it seems to me that is maith le na buachaillí sin peil would be fairly unambiguously "those boys like football".
le + sé -> leis
le na X -> leis na X
|Is maith le||X||Y|
|is maith le||sé||na buachaillí sin|
|Is maith le||na buachaillí||é sin|
Is maith leis na buachaillí sin - "he likes those boys"
Is maith leis na buachaillí é sin - "the boys like that"
an buachaill sin is "that boy". an rud sin is "that thing". é sin is "that (it)", but we just leave the "(it)" out in English.
So when you parse is maith leis na buachaillí é sin, you know that é sin is the thing that is liked, and na buachaillí are the people doing the liking. Because le becomes leis before a definite article, so is maith (le na buachaillí) X becomes is maith (leis na buachaillí) X.
On the other hand, in is maith leis na buachaillí sin, the thing that is liked is na buachaillí sin - "those boys", and they are liked by "he" - is maith leis X, "he likes X".
You can say in Irish : Is maith liom sin = I like that
There are many Irish speakers who very seldom say "é sin" instead of "sin".
Therefore this sentence can accurately be translated (out of context) as: "The boys like that."
Ambiguity in Irish is very common and may be one reason why some poets write in Irish. For example if you hear someone say "an t-úll", they might actually be saying "an tsiúl". Same goes for "níor thuig mé" and "níor shuigh mé".
Lenition, eclipsis and extensive mutation gives great possibility for ambiguity. Put the verb at the start of the sentence -- so that the subject runs into the object -- and you've got a great recipe for poetry. Not so great for technical discussion though.