> Cách a bfuil acat i tig
> etir ith is blicht is mil,
> nocha berair lat ar sét
> in tan racha d'éc, a fhir.

Could we begin to get a handle on the dating by looking at
the verbs?  What do you make of "nocha berair" and "in tan
racha" in this stanza?  Here's what little I can deduce.

1.  nocha berair

The negative "nocha(n)", from OI "nicon", looks like EMI,
but could date to MI.  The verb looks like the modern 2nd
sg. present tense (or subjunctive) "beirir", or possibly
the future "béarfair".  McManus says that the 2nd sg. ending
-ir is not "classical" EMI, although it is often found in
prose texts ("Ní clasaiceach an foirceann -(a)ir sa dara
pearsa uatha táscach láithreach, foshuiteach láithreach ná
fáistineach, cé go bhfuil sé an-choitianta i bpróstéacsanna
na linne..." _SnaG lch. 396).  He gives quite a few examples
of -ir from Maund., CML, etc.  But here we have it not in
prose but poetry.  Where does that leave us?

2.  in tan racha

The stem for the future and conditional from MI onward is
"rag-" and later "rach-, replacing OI "reg-".  A possible
OI form of this phrase would be "in tan regae", I think,
although in theory it might be "in tan no·regae".  MI has
2nd sg. "raga, ·raga".  The 2nd sg. ending for the future
in both MI and EMI is -a.  So this might be as early as
MI or as late as EMI?