If you're planning on a Celtic theme wedding, you're going to have to incorporate some traditions which means some homework for you. If you're heart is really into the theme, then it won't seem like homework. Your guests are sure to love anything you put together. The main attraction, of course, is you and the groom, not the colors and decorations.

Do a few searches on the history of Celtic weddings and find the appropriate traditions and customs you'll want to incorporate into the ceremony itself.
Some links to help you out:
Irish Wedding Traditions
Celtic Wedding Traditions
More Celtic Wedding Traditions

Below you will find some ideas and tips on what kind of flowers you might use, possible colors for a celtic theme wedding, decorating the reception or church and what kind of wedding favors you might want to offer to your guests as a token of your appreciation.

Ideas on Flowers for a Celtic Theme Wedding
Wildflower for bouquets and hair wreaths. You can also use Bells of Ireland, Heather and Thistle.

Ideas on Colors for a Celtic Theme Wedding
White, ivory, green, red, yellow, orange

Celtic Theme Decoration Ideas
Fill glass bowls with water and add four leaf clover floating candles to it. Surround base with flowers or four leaf clover confetti.
Incorporate shamrock lights with the white lights if using any.
Use wildflowers in your centerpieces for the tables.
Baskets full of noise makers as centerpieces. Accent each centerpiece with flowers or shamrock confetti, green, red or silver curled ribbon.
Do include Celtic knots into your centerpieces, table lines and or decor.
Use green glass tealights on the tables.
Use cup cake tree stands holding green iced cupcakes with shamrocks.
Put green or white linens on the table with plaid napkins or use Celtic designed table runners.
Use pewter goblets instead of glasses.
Use pewter candlesticks and flowers.
You can place a cluster of pewter goblets in the center of the tables filled with wildflowers.
You can use tabletop fountains as centerpieces.
As centerpieces, use Irish cookie bouquets.
Use lots of lace linens on the reception tables or place them under your centerpieces.
Use tapestries on the wall if you have them or maybe you can rent them from a local party store.

Entertainment
Bagpiper or Celtic soundboards (kind of like harps) Learn the Irish dance called ceilidh.

Celtic Themed Wedding Favors
Trinity Love Knot Candle Holder favors 
Pots of gold chocolate coins or shamrock bubble gum coins as favors.
Chocolate covered Claddaghs, Horseshoes or Shamrocks
Confetti Flowers which are almond shamrock favors.
Give Irish cookies as favors. Irish made ornaments or horseshoe ornaments.
Claddah candle favors with personalized matchboxes.
Flower seeds  or Shamrock seeds as favors.
Personalized shamrock shot glasses Shamrock suncatchers as favors
Personalized Irish CD's Box of Irish chocolates.
Green Irish fortune cookies

Here are some celtic theme photos to help you come up with some more ideas: