It appears to be the case that UK nationals (including those Irish citizens born before December 1948 who claim their British Subject status) may receive the same benefits as US domiciles. See the US-UK Estate and Gift Tax Treaty 1980 which appears to even apply to Irish residents/domiciles subject to the aforementioned nationality clause:
ARTICLE 8 DEDUCTIONS, EXEMPTIONS, ETC.
(1) In determining the amount on which tax is to be computed, permitted deductions shall be allowed in accordance with the law in force in the Contracting State in which tax is imposed.
(2) Property which passes to the spouse from a decedent or transferor who was domiciled in or a national of the United Kingdom and which may be taxed in the United States shall qualify for a marital deduction there to the extent that a marital deduction would have been allowable if the decedent or transferor had been domiciled in the United States and if the gross estate of the decedent had been limited to property which may be taxed in the United States or the transfers of the transferor had been limited to transfers of property which may be so taxed.
(3) Property which passes to the spouse from a decedent or transferor who was domiciled in or a national of the United States and which may be taxed in the United Kingdom shall, where
(a) the transferor's spouse was not domiciled in the United Kingdom but the transfer would have been wholly exempt had the spouse been so domiciled, and
(b) a greater exemption for transfers between spouses would not have been given under the law of the United Kingdom apart from this Convention, be exempt from tax in the United Kingdom to the extent of 50 per cent of the value transferred, calculated as a value on which no tax is payable and after taking account of all exemptions except those for transfers between spouses.
(4)(a) Property which on the death of a decedent domiciled in the United Kingdom became comprised in a settlement shall, if the personal representatives and the trustees of every settlement in which the decedent had an interest in possession immediately before death so elect and subject to sub-paragraph (b), be exempt from tax in the United Kingdom to the extent of 50 per cent of the value transferred (calculated as in paragraph (3)) on the death of the decedent if:
(i) under the settlement, the spouse of the decedent was entitled to an immediate interest in possession,
(ii) the spouse was domiciled in or a national of the United States,
(iii) the transfer would have been wholly exempt had the spouse been domiciled in the United Kingdom, and
(iv) a greater exemption for transfers between spouses would not have been given under the law of the United Kingdom apart from this Convention.
(b) Where the spouse of the decedent becomes absolutely and indefeasibly entitled to any of the settled property at any time after the decedent's death, the election shall, as regards that property, be deemed never to have been made and tax shall be payable as if on the death such property had been given to the spouse absolutely and indefeasibly.
(5) Where property may be taxed in the United States on the death of a United Kingdom national who was neither domiciled in nor a national of the United States and a claim is made under this paragraph, the tax imposed in the United States shall be limited to the amount of tax which would have been imposed had the decedent become domiciled in the United States immediately before his death, on the property which would in that event have been taxable.