You can view the original register entry for the marriage of Nicholas and Agnes at this link - see the 2nd entry for the 8th September on the right hand page.
The baptism records for the Catholic Parish of Balbriggan are available back to 1770, far earlier than those for many rural Catholic parishes, but no sign of any likely baptism for Nicholas in these that I see. A search of baptisms for the parish between 1845 and 1855 shows at least two children to Nicholas and Agnes - Stephen in 1846 and Christina in 1848.
Widening the search area to north county Dublin and nearby county Meath gives two possible matches for Nicholas - the first in Swords Catholic parish in 1817 to parents Stephen Griffin and Jane Ward - see the first entry for August on the right hand page. The town of Swords, where the parish was based, is in north county Dublin and about 20km (abt 12 miles) from the town of Balbriggan.
The second match is in Ardcath Catholic parish in 1827 to parents Pat Griffin and Catherine English - see the entry for 7th June towards the end of the left page. Ardcarth parish is in Co. Meath and right on the border with Co. Dublin and about the same distance from the town of Balbriggan as Swords.