Donovan of County Cork

After much searching, I've finally found my great x 2 grandmother, Ann Donovan, was born in County Cork 1837. She married John Keenan from County Clare and they went to Wales during the potato famine. Their children were all born in Merthyr Tydfil, Wales from mid 1860's to early 1870's before they sailed to New Zealand. Now I'm stuck - how do I find Ann's birth in Cork?