The baptism records for Blanchardstown Catholic parish go back to the 1770s and I dont see any likey matches for her baptism in the parish either, so maybe your Rose/Roseanne was born in another parish and moved to the area she ended up with Nicholas ?
The difficulty with extending the search area to other nearby parishes is that it makes it difficult to either confirm or rule out any potential matches. The other issue is that some parishes do not have baptism records back to the 1820s. It's also possible that she was baptised in Blanchardstown parish but never written the the register, or on a page that has been since damaged or lost.
A search of baptisms between 1800 and 1840 in Blanchardstown parish shows a few Dowling baptisms that could be connected - e.g. 2 children to a Charles & Mary, abt 10 to Michael & Anne, 4 to Pat & Margaret, 1 to Pat and Anne and 6 to a William and Bridget. There are also a couple of baptisms in the parish for variations of the surname e.g. Doolan, Dooling, Dowlan etc.
Catholic parishes rarely kept registers of death burials and there's none listed for this parish. Civil registration of deaths started in 1864, and register images back to this date are due to be added to the free IrishGenealogy website later this year. Images of the registers for 1878 onward are already available on the site.