Carol
Just got some papers from PRONI on Archibald Grissam (d.1871 aged 35, batchelor) which may shed more light on your Sarah's ancestors. Archibald is the son of Hugh Grissam of Ballygawley and Ann Taylor. He died without making a will but the administration of the estate was granted to his brother William, the beneficiaries being William, John and James Orr Grissam.
William (m Nancy Duff) and John (married Elizabeth Finlay) were definitely Archibald's brothers and he was a batchelor so it is a fair bet that James Orr Grissam was another brother as having an equal claim with John and William. As Sarah's father was James, this could be the guy!
This fits with some of the children's names as, by the above argument, James had a brother William and Martha and Hugh were the brother and sister of his father.
I know it's fairly tenuous but given the rarity of the name it must be a real possibility.
