Hi, not much information there, more information may yield better results.
I note several Millars recorded in Ardstraw in 1796, Andrew, Edmund and John all appear in the Flax Growers bounty list for the Parish but no David.
There is a David Millar recorded in 1833 in the Tithe books for the Parish of Ardstraw living in the townland of Churchtown, but this may be too late for your David.
Have you dates for marriage, children etc?
Regards
Joe