Nick,
Go with what Seosamh suggested:
Is tibhe fuil ná uisce. Word for word it means "is thicker blood than water", a good, direct translation of the English. For a native proverb with a similar meaning, see this one in the Dáltaí collection:
http://www.daltai.com/proverbs/weeks/week82.htm Tá sé seo le fáil in FGB (Ó Dónaill) freisin, s.v. "gaol", leis an aistriúchán "blood is thicker than water".