It's "일 것이다." And I feel "일거라고" sounds like a young kid around, maybe 8? But it's totally accepted in any other less professional, less formal situations. And "일 것이다" is a future tense of "이다." (this can be used as a future tense, but works also as a prediction. ie: 내일은 맑은 날일 것이다.)
I guess this can be broken down into "이다" + "것이다" But I don't really feel it's a combination of those two, but just one bunch used as a inbreakable unit.