I have not seen "seanmas" in any other context but could understand it from the title of the thread.

The -s could be an ending to form a second (abstract) noun from an already existing one, in the same way as adding -ship forms musicianship from musician. Seinm is the act of playing so that seanmas would be some more abstract noun relating to the same.

Deriving an abstract noun from the genitive form of a noun is not irregular and seanma is a genitive form of seinm.

As Dinneen gives an adjectival form "seanmach" which means 'tuneful', seanmas could also be formed by removing the adjectival -ch and replacing it with the abstract -s giving "tunefulness"

In my opinion, you are on safer ground with seanmas wherever it was found.