Seo in this position implies 'is' as well as 'this'. 'This' in the sense of 'this grandfather' needs the definite article as well as 'seo' - 'an seanair seo'. Here the 'seo' in conjunction with the 'an' does not also imply 'is' - so 'seo seanair math' = this is a good grandfather, and 'tha an seanair seo math' = this grandfather is good.
seanair always starts with a "sh" sound - sheanair always starts with a "h" sound.
seanair is the basic form (or nominative case) of the word for "grandfather". Most of the examples so far have been sentences where we are talking to a grandfather, in which case we had to use the vocative case, adding the vocative particle a and leniting the plain form of the noun, giving us a sheanair.
In this sentence we are talking about a grandfather, so we need to use the nominative case (or the "plain" form), seanair.
se and si both have a "sh" sound, and sa, so and su have an "s" sound - when lenited (sha, she, shi, sho, shu) they all have a "h" sound.