Your Patrick was born well before the start of civil records, so there will be no birth cert, but there could be a record of his baptism. Dunn[e] is quite a common surname in the area around Co. Kildare, so you probably would need a bit more detail, and more definite details on his parents, to check if any baptisms you find were in fact your Dunns..
Robertstown is a town and townland in the north of the county - so maybe Robertstown farm was somewhere nearby ?
A quick search of the free index on RootsIreland shows about nearly 50 baptisms for the name Patrick Dunn[e] in Co. Kildare around the right time *, and about 11 in Allen/Milltown RC parish, which covered most of Robertstown/Killmeage parish.
(* You have to allow a few years either way in searches since ages given on census, death records etc are not always accurate. )
Do you have a marriage cert for Patrick ?
Shane