As a general rule it's it very hard to enter any sector as a business that you have no experience of working in as an employee, so if you spent time working for O'Brien's or Finlaters etc then you know what to do but even if you are experienced at importing other goods I would still be very cautious about doing what you suggest.
Do you have access to a distribution network?
Have you approached either a supermarket buyer or a high end off-licence such as Redmond's in Ranalagh to see if they will carry you stock?
Is it high end or low end (commodity) wine? for your purposes low end is anything that will retail for under €30 a bottle and as a small importer you really need to import high end wine from a small vineyard, i.e. €100 + a bottle from a producer who it too small to attract a large importer. This sort of thing is common in France but no so common in California.
Also remember that wine doesn't like being moved so it's not cheap to transport properly.
Even if you have, as a new customer of the supplier in the US you will have to pay for goods up front that you mightn't sell for 6 months or a year. Therefore you will require a bigger mark up than an established importer/ distributor who has his channels set up.
Sorry to sound so negative but no business is simple and in a well established sector the margins will not be high so good supply channels and the ability to get a fast return on investment in order to free up capital and minimise short term borrowings can make all the difference.