Unfortunately there are gaps in the baptism records for Upper Creggan. The records cover 1796 to 1803 and 1812 to 1829 then 1845 onwards.
So the reason you cannot find Ann’s baptism in 1843 and that of any siblings born between 1829 and 1845 is because sadly the records don’t exist. (They have either been lost or were never kept in the first place).
Here’s a link to Patrick’s baptism in October 1826:
The records that do exist for the early 1800s are very limited as you will see. Basically the couples names, the date and the sponsors (or witnesses for a marriage). There’s usually no way of telling where they lived and who they were related to. Richard Burns & Mgt McEvoy could be a different family. Your family may have been born in the years for which there are no records or in a different parish. (If they were labourers, they often moved around to follow available work).
Research in Ireland becomes very difficult around 1800 due to the general lack of records.