Brosnahan, Mahoney, and Healy - Arabella/Gortagullane townlands

I am trying to find the family link between Deborah Healy nee Brosnahan of Arabella (Griffiths valuation) b.1824 - 1899 and Johanna Fitzgerald nee Brosnahan of Gortagullane (wife of John Fitzgerald, Griffiths Valuations) b.1817 - 1882 and a niece of Deborah Healy named Mary Mahoney of Arabella b.12 September 1847 - 1944.

1. Mary emigrated to New Zealand in 1861 (aged 14) with Johanna Fitzgerald nee Brosnahan, other Fitzgerald family members and Patrick Healy - Deborah's son. Mary Mahoney was born at Arabella according to her family tree. She married Thomas Shea at the young age of 17 and had 15 children. She was 96 when she died. Her parents are unknown but her second daughter was called Ellen so this may be a clue to her own mother's name. First daugter was Catherine which was her mother-in-law's name.

2. Deborah was widowed prior to leaving Ireland in 1861 and her parents were Michael Brosnahan and Mary Galvin according to a direct descendant. She remarried in NZ and the farm she and Thomas Shaw then worked was called 'Arabella'. Her sons John, Michael and Patrick Healy also emigrated to NZ. Her first husband was Edward Healy, son of Patrick Healy and Mary Brien who had a connection with Annascaul, Dingle.

3. Johanna Fitzgerald nee Brosnahan was the daughter of John and Mary Brosnahan (from death cert.).

4. The townlands of Arabella and Gortagullane have a common boundary.

I suspect Deborah and Johanna were cousins and that their fathers Michael and John were brothers (but would like proof). Deborah and Johanna settled is the same rural district of Marlborough, NZ. 

I have searched through birth records but have yet to find anything for any of these three women, let alone that elusive link.

Any help would be very appreciated. Thanks for reading.