Is it true that if we vote for the Lisbon Treaty the EU can instruct Ireland how much we spent on defence and on weapons?

NO it is not true. WE won a guarantee on this and if we vote YES that guarantee will have the force of an international agreement. The guarantee provides that the Lisbon Treaty:

“does not affect the right of Ireland or any other Member State to determine the nature and volume of its defence and security expenditure and the nature of its defence capabilities.“