You can search for the GRO details on freebmd.org.uk However, there are 2 Mary Ann KELLYs born in the area:
Sep 1879 Wigan 8c 147
Dec 1879 Leigh 8c 253

If you are not certain which is correct, you can order the certificate but without the full GRO reference. Just use the year and state that the mother's maiden name is FALLON. It will take longer to get the certificate (3 weeks instead of 5 days to issue, + postage time).