The recording of baptism and marriage for the combined parishes started in early 1832 just a few years after Catholic emancipation. After 1839 Mullagh (Kilmurry-Ibrickane) kept their baptism and marriage register.
If searching church records in the Mullagh (Kilmurry-Ibrickane Parish) and your family was living there in the 1832-1839 period you may want to look at the Miltown Malbay (Kilfarboy Parish) and see if they are listed in the Miltown Malbay record.
My g-g-grandmother, Bridget Gorman of Shanaway was baptized in 1834 along with her twin brothers in 1832. I found them listed in the Miltown Records this added one more generation to my family tree. The early records that are on microfilm are difficult to decipher but a careful search can bring rewards. Most baptisms have the townland listed but very few with sponsors.
I have noticed that if you are using the indexed copies from the Miltown Malbay Parish that the parish offices now have and permit genealogists to view, you will find that there are families listed with baptisms taking place between 1832 and 1839 and then stop. This is a good indication that this family may have lived in a townland that joined the new Mullagh Parish in 1839. If there is a family in the index you suspect may be yours, you will have to look at the originals to see what townland the family was living. The indexes do not have the townland listed. The same goes for the Mullagh Parish. If the baptism records start in 1839 the earlier records may be in the Miltown Malbay records.
I found a number of families from the Kilmurry-Ibrickane (Mullagh) Parish having baptisms in the 1832-1838 Kilfarboy (Miltown Malbay) Parish registers and later in the Mullagh registers that start in 1839. Even after 1839 some families continued to go to Miltown Malbay for a while.