One of our commenters [T&J] couldn’t wait to comment on this.. But during NI Questions today, the DUP’s Peter Robinson asked the Secretary of State for Wales etc, Peter Hain, a question about the Sewel Convention – Normally associated with the Scottish Parliament, it “gives a broad statement of principles for relations between the executive authorities in the UK, Scotland, Wales and Northern Ireland.” and, in this case, any proposed Irish Language Act.From Hansard
3. Mr. Peter Robinson (Belfast, East) (DUP): If the Government will adhere to the Sewel convention in relation to Northern Ireland legislation if powers are devolved to a Northern Ireland Assembly. 
The Secretary of State for Northern Ireland (Mr. Peter Hain): The UK Parliament retains authority to legislate on any issue, whether devolved or not. However, the Government would not normally bring forward legislation with regard to devolved matters except with the agreement of the Northern Ireland Assembly.
Mr. Robinson: I am grateful to the Secretary of State for pointing out British sovereignty in Northern Ireland and the convention that would be followed. Will he confirm that the Irish language is a devolved matter and that there is no prospect of the Government introducing legislation before the date that he set for devolution? Will he also indicate that the Government have no intention of breaching the convention if there were a devolved Parliament, which would therefore decide the matter, and that, if the Government were to legislate, they could do that only with the Assembly’s agreement?
Mr. Hain: I am happy to confirm that it is a devolved matter, provided that there is something to which to devolve it and that there is an Assembly up and running from 26 March. As the hon. Gentleman knows, we will shortly announce a consultation paper, which will explore several options on a way forward. That consultation will last three months and we will have to see where we go from there. There is no slot in the Queen’s Speech for rushing through emergency legislation before 26 March. Assuming that restoration happens—as I hope that it will—on 26 March, of course it remains the case that, although Parliament is sovereign, we would not legislate on a devolved matter, except with the Assembly’s acceptance and agreement.[added emphasis throughout]
More details from the House of Commons Library [pdf]
Outline of the Sewel Convention
The “Sewel Convention” is a colloquial term for the UK Government’s stated policy on legislation concerning devolved matters in the UK Parliament. It is named after the Government Minister, Lord Sewel, who set out the terms of the policy in the House of Lords during the passage of the Scotland Bill 1997-98 on 21 July 1998:
Clause 27 makes it clear that the devolution of legislative competence to the Scottish parliament does not affect the ability of Westminster to legislate for Scotland even in relation to devolved matters. Indeed, as paragraph 4.4 of the White Paper explained, we envisage that there could be instances where it would be more convenient for legislation on devolved matters to be passed by the United Kingdom Parliament. However … we would expect a convention to be established that Westminster would not normally legislate with regard to devolved matters in Scotland without the consent of the Scottish parliament.[original emphasis]
The Sewel Convention is just that: a convention. It is not enshrined in the Scotland Act 1998. However, it was embodied in the Memorandum of Understanding between the UK
Government and the devolved executives, which was drawn up in 1999.2 The Memorandum gives a broad statement of principles for relations between the executive authorities in the UK, Scotland, Wales and Northern Ireland. The Memorandum is not intended to be legally binding, but it does represent a political undertaking.
The thinking behind the Convention is that the UK Parliament, as a sovereign body, retains full legal power to legislate on devolved matters, yet the spirit of devolution implies that political power rests with the Scottish Parliament. In order to avoid conflict, the Government undertook not to seek nor support relevant legislation in the UK Parliament without the prior consent of the Scottish Parliament. This consent is embodied in a “Sewel motion,” or, formally, a “legislative consent motion.”
The Sewel Convention applies when UK bills make provision for a devolved purpose (ie a matter on which the Scottish Parliament is competent to legislate), when they vary the legislative competence of the Scottish Parliament, or when they vary the executive competence of Scottish Ministers.