I came across a William Fyans in the Parish Register for St Michan's Church in Dublin. He was born too many years ago to be your grandfather, but you might be interested anyway.I also read somewhere that a Fyans was mayor of Dublin, and there was a Fyans castle now renamed.
On Aug 9, 1696 a William Fyans was baptized -- son of Henry Fyans, porter, and Margaret, his wife.
St Michan's is one of the oldest churches in Dublin.I took a few pictures of it and put them on Webshots.
I have information on a few more Fyans from Rush, which I will post here for you, from the 1800s.Rush (north of Dublin) is home to a line of Irish Fynes, and it is interesting to see so many Fyans in the same small village.Makes me wonder if these Fyans were Fynes who spelled their name differently.