Finding McKittrick Ancestors in Ireland

Hi I have come up against a brickwall with my family tree of William McKittrick of Dundonald, outside Belfast, Ireland, circa 1860's I believe his father's name may have been John. William had a son David about 1848, who married Catherine McConnell, they lived in England and had a large family, David returned to Belfast in the 1890's after the death of Catherine and with a new wife Agnes. I cannot get back further than William who may have born about 1820's or find who he married and what siblings David may have had. Any help would be appreciated.

Eleanor