What do there two sentences mean:

tá seo le rá faoi
tá sé sin le rá faoi

My attempt at it:

This is what is to be said about it.
That is what is to be said about it.

Why "sé sin" instead of sin?

You are probably referring to:
http://www.focloir.ie/en/dictionary/ei/say-this-much

I have only a hazy notion:

You could perhaps use both (seo and sin or sé seo and sé sin) but:
Both sentences are used in different situations (as it is on focloir.ie).
sé sin is referring back to something already mentioned. So, an anaphoric pronoun sé is useful.
Seo is referring forward, cataphoric: Something following. Moreover, it is not only referring but rather pointing to the things to be said. The whole phrase is like an imaginary index finger, the finger tip is perhaps a double dot: Tá seo le rá faoi: ...
If you point at something there's no need for sé, simple seo is enough.

Tá seo le rá faoi; Tá sin le rá faoi:

I'll give a stab at it:

The omission of "sé" in both sentences may be due to the use of a higher register of language; almost a poetic device.

Tá seo (a leanas) le rá faoi
Tá sé sin (atá ráite agat) le rá faoi

_________________
[hr]Múinteoir Gaeilge - Irish Teacher[/hr]
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).