Thankfully, some schools have moved to a long white robe, small rental charge, or school uniform for ceremony.
DD1 received FHC the year before Dunnes Stores started selling FHC dresses. We got her dress in a local school uniform shop, they had a fairly extensive choice and we went for a simple style. She wore her dress for the entire day, got chocolate on it, I washed it in the washing machine that night, was up early to iron it and she wore it to Sunday Mass and again to school on the Monday for school photos. She also got to wear it in the school May procession and as an angel in the school Nativity. Two years later her sister wore it, same events. So it got a lot of outings. DD1 was dressed top to toe 10 years ago for much less than €200. DD2 got new shoes from Dunnes, ballerina flats for €6 and a new veil as my aunt wanted her to have something new and had given us some money. Same aunt knitted a lovely white cardigan for DD1.
Dunnes Stores now do lovely FHC outfits for both boys and girls. Follow them if you are on Facebook and you will see when they start to come into stock. If you go for Dunnes, someone else is likely to have the same dress, remember this can happen with a very expensive dress too, children will think it is hilarious!
For our family celebration we had a low key party at home, focus was on nice food for our guests and we were lucky with the weather and we have nice garden space. There were no caterers, no entertainers, no bouncy castle and no decorations. I think I got girly pink floral paper napkins for the occasion, I'd have had to buy some anyway. Both were special family days, a lot of hard work for me but so worth it for lovely memories and photos.
Can you tell us your nearest city or town OP, someone may have recommendations for an independent shop. Or, charity shops often have lovely dresses available. My tip is to visit the shop yourself first, if the prices are beyond your budget do not take your DD there. And remember you don't have to buy accessories or shoes from the dress shop, they are vastly overpriced.
Veils are not compulsory, but a little plain one is a lovely finishing touch. Please don't go for a tiara, they can look odd on grown adult brides and positively ridiculous on little girls. Short sleeves are the way to go. Skip the bolero, but maybe have a little white cardigan from Dunnes or Penneys for the early morning chill. You definitely don't need - handbag - parasol - gloves - tiara - white watch - all of these are considered fidget accessories and may be banned from the church by the school. Doting grandparents and godparents will be asking you nearer the time what they could gift your child, have a serious think about this - perhaps a cross and chain, a first watch, not necessarily white that can be worn after the day, a veil or shoes. Otherwise you may get something personalised that is not to anyone's taste.
Did you like DN's dress? I'd have been delighted to pass our dress on to a family member. Ask your dSIL.