Ive worked in entertainment (Wedding DJ) for last 10 years and attend between 30-40 weddings a year. The below is based on what Ive seen and after talking to guests and bride and grooms
Food. beef or salmon/corn fed chicken etc instead of turkey and ham can make a difference of 1500e (10e extra for 150 people). People are hungry with a couple of drinks on, theyll eat anything.
Buses. Is it 300e plus per bus? Ive seen 3 dublin buses lined up to take 4 people home. Forget about them unless you know for a fact therell be lots (50-60) going to the afters.
Toast. this is expensive costing up to 6-700euro to buy everyone a drink. you dont need it. everyone has a drink in their hand and will just order more for the sake of it.
Cars. c500e to get picked up. Hire a ferrari and drive it yourselves. still cost you a few bob, but you get to drive it!
Country weddings for dublin people. "Oh honey, lets share our day with everyone we know by having our wedding in a lovely castle in cashel". Average couple before new clothes and pressie now has to spend 200e on accomodation and arrange babysitters and transport. No matter how scenic you think your wedding day looks in that castle, half the people there will be looking at you and thinking "wtf did they make us pay all of this money for, why didnt they just get jurys?". If you wanna share your day, have it central to the majority of people who are going to it, this has been said to me by a LOT of people.
same goes for foreign weddings, wanna go foreign? invite close family and have a party when you get back. People love a day out with friends and family, not a week.
fireworks. forget them. youll spend 2-300e plus on some bangers. Unless you get the lads who do skyfest, theyre a waste of money. Noone will say this to the brides face though.
Pipers. these are so-so, they can add to the occassion but more often then not dont.
Items Ive seen that just always seem to work:
chocolate fountains.
chicken and mushroom vol au vents for starters.
finger food during the night. Make sure this is good and plentiful!!
really good entertainment, from DJs to bands to church singers to magicians to clowns to irish dancers, Ive seen them all and people remark on them long after the weddings more then anything else. Yes, I do work in entertanment so I may be biased but Ive seen couples save a couple hundred euro and get a not-great band and have people in their chairs all night admiring the 2000e the bride spent on wedding favours ( chocolates, hand tied dinner menus, chair bows, little stones imported from egypt with the bride and grooms names written in chinese etc, etc, etc).
anyway, my 2 cents. (oh yeah, and pay the DJs more money
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-- having reread this and other posts, you get the gist, lots of food and music. save on the rest and spend it on your honeymoon.