We went to Ireland for 5 days for a wedding with a 9 week old in November.
She slept in her carrycot attachment for the pram (took spare sheets), also took her bouncy chair and changing mat. I wouldn't have bothered with the chair and mat, as they went used. There were too many family members desparate for a cuddle.
I exclusively express so had to juggle keeping milk cold, and me expressing. We did buy little 200ml bottles of readymade formula to ensure she could be fed. A very cold bathroom windowsill in Wales and a bag of ice from Bangor Tesco got us through. I had a pump and a load of sterilised bottles in a little square cool bag, and then the other bottles with milk in in another. Took a couple of froze bottles too. We took our big cold water steriliser, and bought the single bottle one too just for years so we could use the flat lids as more stable. We also have the travel warmer (all tommee yippee).
I took little travel size bottles of shampoo/soft wash/baby bath, and most of it came home. I didn't need 3 towels for her!
Clothes wise, I wrote a list of days and matched up clothes (trying to make sure those bought as gifts were warn when the person who bought them was there) plus a baby grow and vest for the night, plus a spare vest and baby grow for each day. Brought a load home clean, as she's not a sicky baby.
Muslins, bibs and cloths I just chucked a handful in. We took a pack of aldi size 2 nappies (44?) and a pack of pampers size 2 (20 odd?) as people said they held more so thought they would be better for travelling. Didn't like them, they smelled funny, just the aldi ones would have been fine.
We took a couple of packs of baby wipes, and bought a pack in Dunnes after an explosive poo when we only had a quater of a pack left, in the knowledge they sold nappies, but not wipes on the ferry. Didn't bed them in the end.
We didn't take a baby bath as she either came in the bath with me or we used one we borrowed in Ireland. I also packed sample size wash powder and fabric conditioner of the type we use in case we needed to do a wash, but didn't need them.
We were lucky that we were staying with family, and several extended family and friends have babies so we could have borrowed anything we needed or bought it - as my MIL said, they do have shops, just not 24h ones like at home!
We're looking forward to going back next year for the next wedding 🙂