There isn't a direct, single translation of "Sullivan" in Irish Gaelic. Irish surnames often have multiple possible spellings and origins, depending on the specific family history. However, "Sullivan" is derived from the Irish Ó Súilleabháin (or similar spellings).
Therefore, you'd write it as Ó Súilleabháin. Other variations might include Ó Súilleabháin, O'Suilleabhain, etc., depending on the Anglicization used by the family.