Hello Yvonne,
First I would say take the date 1849 with a large grain of salt. People generally hadn't a clue of their birthdate or how old they were, and when asked at points of arrival usually took a guess .
Generally speaking church records are between two sites. rootsireland.ie have most of the counties but for the ones that they dont have the government set up irishgenealogy.ie This latter site covers Dublin. You need to imput your g grandfathers name starting about 1844 or so and gradually go up in years. I found an entry for a Thomas Archer in 1853. This may or may not be your man.
It's important to try verify records by using other information. Have you a mother's name or fathers? Many children were called after grandparents. So any of the names on the record match any in your family??
Can you go to the migration records and try find out more about his arrival? www.immigrantships.net?
I know Australia kept fantastic records of arrivals and many NZ passengers disembarked in Australia first.
Happy hunting,
Anne