Hi Auryn,
The translations from Google Translate aren't good I'm afraid.
For example spell is translated as litriú which means spelling (of a word). The Irish words for spell (as relates to magic and enchantment) are draíocht or geis, and ortha if you're talking about an incantation or magic charm. Once is translated as uair amhain, which means once, as in one time. That's just a couple of translation errors. The grammar is all over the place.
These would be my own translation attempts...
"Sisters of the tides are we, bound by sand, and salt, and sea. Selkie, mermaid, siren, daughters, priestesses of the holy waters."
Siúracha na dtaoidí muide, gafa le chéile ag gaineamh, salann is sáile. Maighdean róin, maighdean mhara, Síréana, iníonacha, bansagairt na n-uiscí naofa.
I've use siúracha for sisters. It suggests sisterhood - kinswomen rather than blood sisters.
I've used sáile instead of farraige for sea. Sáile is salt-water or sea and the s sounds in salann and sáile are in keeping with the sounds in the original English version.
Selkie is an anglicization of Scots Gaelic. The word doesn't exist in Irish (as far as I know) so I've used maighdean róin (seal maiden) which does exist in Irish (as far as I know), and is basically the same concept as a selkie.
Síreana is the mythologocal siren.
Bansagairt means priestesses, but you might also consider bandraoithe. Draoithe are duids, magicians, diviners. Bandraoithe are the female versions of same.
Couple of small corrections, Dale.
Taimíd deirfiúracha... should be Is deirfiúracha... (copula)
Cheanglaíomar le gaineamh... means we were tied with sand... You could use ceangailte le chéile ag....
Cluanaí can be translated as siren but the meaning is more along the lines of seductress than mythological siren.