Here are some Korean phrases you might like to tweet to her or post in her fancafe as support!

생일 축하해요! = Happy birthday!

생일 미리 축하드릴게요! = Happy birthday in advance!

생일 축하해요, 아이유씨! = Happy birthday, IU!

23번째 생일 축하해요! = Happy 23rd birthday!

아이유씨 생일 축하해요!! 올해는 좋은 일만 가득하시길 기도합니다 = Happy birthday, IU! Wishing you all the best for this year!

생일 축하해요, 언니!... 항상 좋은일만 가득하시길 빌게요! = Happy birthday, Unnie!...wish you the best!

저희는 (country name)에 있는 팬들입니다. 생일 축하드려요! = Your fans from (country name) wish you a happy birthday!

생일 축하드립니다! 멋지고 아름다운 생일 보내세요! = Happy birthday! I hope you have a great and beautiful day!

생일 축하합니다! 늘 에너지 넘치게, 귀엽게, 그리고 건강하게 사세요! 좋은 일들만 가득하시구요! = Happy birthday! Stay strong, cute and healthy! Wishing you all the best!

언니, 생일축하해요! 이 특별한 날에 정말 좋은 시간 보내세요. 매일 매일 축복이 가득하시기를 = Happy birthday unnie! Hope you'll have a great time on such a special occasion. I wish each day is a blessing for you.

(For those that share #IU's birthday, May 16th....) 우리가 생일이 같은날이라는 것을 알았을 때 정말 기뻤어요. = I was really happy when I found out that we have the same birthday!

(For those that want to put in the name of the person you ship IU with....kekeke) 안녕하세요, 언니! 오늘은 언니의 날이죠, 맞죠? 생일 축하해요! (name)씨와 그리고 가족분들과 즐거운 축하시간 가지세요. = Hello unnie! Today is your day, right? Happy birthday! I hope you'll have a great celebration with (name) and your family.

(For those of you that forget to wish IU a happy birthday on May16th....) 늦었지만 생일 축하드려요. = I know I'm late, but happy belated birthday.

source: link (This is a REALLY cool site, run by @KoreanHelp on Twitter! They also do personal translation from Korean to English and visa versa! You should join the site and, if you speak Korean, HELP THEM!)