Great Seal of Northern Ireland now used for all matters in Northern Ireland for which Great Seal of Ireland was formerly used: Irish Free State (Consequential Provisions) Act 1922 (13 Geo. 5 Sess. 2 c. 2), Sch. 1 para. 2(4)

Functions of Privy Council of Ireland transferred to Privy Council of Northern Ireland: Irish Free State (Consequential Provisions) Act 1922 (13 Geo. 5 Sess. 2 c. 2), Sch. 1 para. 2(1)–(3)

http://www.legislation.gov.uk/aip/Geo3/40/38/section/XAct of Union (Ireland) 1800An Act for the Union of Great Britain and Ireland.1978-02-01texttext/xmlenStatute Law Database2011-05-261991-02-01

X Great seal of Ireland.

And the great seal of Ireland may, if his Majesty shall so think fit, after the union be used in like manner as before the union except where it is otherwise provided by the foregoing articles, within Ireland, and his Majesty may, so long as he shall think fit, continue the privy council of Ireland, to be his privy council for Ireland.