a little more explanation of Irish grammar and spelling/

The word "séard" is derived from "is éard" which in turn derives from "is é an rud" (it's the thing [that], thus)

Is, also 's (ba or b' in past and conditional) is a connective word, known as the copula. I've seen it described as a "defective verb". It is used for statements of identity such as "is bean í", she is a woman; and in a lot of common phrases such as "is maith liom" (I like, literally, it's good with me) and "is mian leo" (they want, "mian" = a wish, a desire). It can also be used for emphasis. "Is lá breá é", it's a fine day or "Tá lá breá ann", there is a fine day in it = it's a good day; while "Is breá an lá é" emphasizes the word "breá" (fine,lovely), It's lovely is the day.

The word "agus" means "and". It is often abbreviated and is written as 's or 'us or 'is or simply is. I think that must be confusing to some people when looking at/listening to Irish language lyrics.