In accordance with Article 50, paragraph 3 of the Treaty on European Union, the EU Treaties shall cease to apply to the United Kingdom from the date of entry into force of the Withdrawal Agreement or, failing that, two years after its notification of its intention to withdraw, unless the European Council, in agreement with the United Kingdom, unanimously decides to extend this period. In the case of the UK, at present, this means 11 pm on 29 March, 2019.
Any request for an extension would need to be made before then, and would have to be made in sufficient time to allow for a decision to be made by the European Council in accordance with the terms of Article 50, paragraph 3. Ireland would be open to such a request. As President Tusk has said, should the UK make a reasoned request for an extension under Article 50, the other Member States would stand ready to consider it.
Any decision to formally request an extension of Article 50 remains a matter for the UK.