You can have a lot of fun with it. We worked a couple of themed cocktail party - martinis, champagne, cosmopolitan, tequila and alcohol are all common themes to choose from. What for me is Hennessy, who also the part of Ireland and to integrate into your invitations, decorations and food choices. There are plenty of online resources for planning or St. Patty's Day Irish themed party - complete with menus and decorating ideas - and you could easily incorporate that in yourHennessy themed party.
I have some great ideas for your invitations. Of course you could always do an Irish themed invitation (green, shamrocks, Celtic imagery, etc). But for something really creative, you could do an invitation incorporating a Hennessy bottle or snifter glass. You could print the invitation information on the label of the bottle.
Wine Charms Personalized
This could either be a picture of the bottle/snifter either mounted to or printed directly on some nice cardstock. If you want to get even more creative, you could cut the invitation out in the shape of the bottle/snifter. I think this would be really cute. For something really spectacular, you could purchase mini bottles at a container store and create your own custom label for the bottle or do a variation on the popular "Message in a Bottle". The invitation bottle would then be sent in a shipping box instead of an envelope -- very unique and truly memorable.
You should check out search engines for some great ideas about Hennessey. Often times, you can find a website with a good Downloads section and some very cool recipes you can serve at your party. You could also incorporate this into your invitation and send a recipe of one of the fun Hennessey drinks such as Hennessey Brown Sugar with the actual invitation.
A nice touch for your guests and a good way to introduce the theme. Then you can play this up on the invite wording by saying something like, "A Recipe for Fun - Ingredients: A Beloved Birthday Boy, Several Great Friends, and Delicious Food and Drink; Directions: Mix Well and Serve Chilled on Friday, May 23 at 7 p.m. at Tessa's House (replace with your party details). Would be very cute and easy.
You can also find websites with 'evites' that you can send. These are very cool. I'm a stationer so I'm biased, but I recommend you also send snail mail invitations. It makes your guests feel special when they receive something other than virtual mail. And it really makes the event seem more important. You may want to design your invitations to coordinate with the evites available online. That gives you another invite option.
Decorating/Favor Ideas:
Set up drink accessories. These are the memorable touches that will leave your guests "ooohing" and "aaahing." Have plenty of kitschy items like plastic swords and paper umbrellas on hand. Cut up different types of fruits and vegetables and place colorful toothpicks in them. Of course, have a wide array of funky glasses available. Bar towels, an ice bucket and tongs, and cocktail shakers are other good ideas.
Depending on the size of your guest list and budget, I think personalized or engraved snifters would make excellent favors. Another idea are the popular wine charms popular at parties to allow guests to easily identify their glass. These are pretty inexpensive and are functional for the party, as a well as a great takeaway for your guests. Another idea is to give guests liquor filled chocolates - these are really popular and quite tasty. And would also go with your theme.