Records are recorded by parish - so the marriage probably took place in the parish church for Casteltown/Kilpatrick. If there was a chapel of ease in the parish at the time and the marriage took place there, then the details would have been forwarded to the parish church to record in the parish register.
I am fairly certain that the RootsIreland webpage shows the denomination of the record you have paid to view, but it also allows you to search by parish. The denomination is shown in brackets after the parish name - i.e. RC=Catholic, CI=Church of Ireland, ME=Methodist etc
Church of Ireland records for Castletown only go back to 1884 on RootsIreland, so if the marriage in question took place before this then it must be Catholic. Another way to tell for baptism records is that in general Church of Ireland baptisms dont show mother maiden surname, whereas RC baptism records do.
As GerryS mentioned above the St. Patrick's being discussed earlier in this topic is Church of Ireland. The RC church could be dedicated to a different saint. Note that this was not a constant name, as a church could be rededicated under a new name after renovations or rebuilding.
The RC Parish church was located in the village of Castletown, which is located close to the road between of Navan (the county town) and Nobber. You can see the location of the RC Chapel on the 1st Edition Ordnance Survay of Ireland Map :
Castletown RC Church (select the Historic 6" maps for the 1820s/40s view)
You can see the current church at the same location on Google Street View -
Castletown, Co. Meath RC ChurchShane