eDIL has several references to Meyer's line in the poem and the word "hsamhain" but that could simply be one scribe (eDIL) copying the error in the transcription of another scribe (Meyer) without going back to the original manuscript to check.
My simple answer to the question of your final sentence is, "don't know."
From: Liz Gabay <[log in to unmask]>
To: OLD-IRISH-L <[log in to unmask]>
Sent: Saturday, July 09, 2011 at 7:30 pm
Subject: Re: [OLD-IRISH-L] 'Somhain so' 7 & 8
Apparently I didn't phrase my question clearly, but I was not wondering
about the appropriateness of the lenition in this phrase. Rather, I was
questioning the representation of the lenition by 'hs' in the word 'hsamhain'.
Lenition is usually represented by 'sh' or a dot over the 's'.
I wondered if 'hs' is seen in other manuscripts, or if it could be a typo in
Meyer's transcription? Liz