I’m grand

..can have many hidden meanings. What it may mean to the speaker is,

Well, there’s a lot going on but nothing I want to discuss right now.

Whereas the recipient might infer,

Ask no more questions unless you have half a day to listen to me.

Or, it could simply mean that the person is, in fact, grand and the recipient will interpret this correctly if it is delivered with just the right amount of cheer.

Another common use of the word, and it is usually offered after the speaker has turned down the offer of assistance. For instance, a shop keeper might ask you if you need help selecting items from the shop. If your reply is that you’re OK and can select the items yourself, his response might be:

Ah, grand so

It is usually delivered with an air of nonchalance and it probably means that although he is relieved that you don’t need his help he is also slightly insulted that you haven’t accepted his offer.  It pretty-much closes off any further offers of help, unless it is requested,  in which case, the help will delivered in a slightly less friendly manner and you will never gain the full benefit of his expertise that you would have had you allowed him to help you in the first place.

However, if, in the above scenario, you had answered the shop keeper’s initial question with

Ah no thanks, I’m grand

…that would have been that, except in the case of the shop keeper not being Irish and only in the process of learning English. He may have wondered what you meant, and, with all that time he has on his hands not having to help you, gone straight to his English Dictionary to look up the definition of this word ‘grand’.  So the next time someone asks him how he is, he will be able to reply with confidence that he is:

large or impressive in size, extent or consequence, characterized by and attended with magnificence or display,  sumptuous, of great distinction and pretension,  dignified and haughty, designed to impress, very good, wonderful, comprehensive, complete, worthy of respect, fine, large and impressive in conception and execution, most important, chief.