Because the number goes to the ten-thousandths place, and that is where Irene rounds to, Irene will end up with the given number. Since we are looking for the largest number in the group, anyone who ends up rounding down is not the winner because Irene has a number greater than them. Thus, we can ignore all people who end up rounding down. When we round a number, we look to the digit to the right. If the digit is less than 5, we round down. Thus, if we round by looking at the 2, 3, or 4, we will round down. Thus, Alice, Bob, and Carol will round down, so they are not the winner. Devon will round to the nearest ten. Because 5.6789 is greater than 5, Devon will round up to 12350. When we round up, the most we can increase the number by is by increasing the decimal place we are rounding to by 1. For example, if we are rounding to the tenths place, the most change that the tenths place can undergo is increasing by 1. We cannot increase it by 2 through rounding. Thus, when Eugene rounds to the nearest one, the highest the ones place could be is a 6, and the tens place will still be a 4. Thus, Eugene's number is smaller than Devon's. Likewise, all of the other people will round their numbers up by less than Devon has, so $ \boxed{\text{Devon}} $ is the winner.