이러다, and its forms 이러면서 and 이러고, means 'goes like this' — it quotes what somebody said or thought. The -ㄴ데 in front is the trailing 'but...' that leaves the thought half-voiced, so 이것은 좀 아닌데 이러면서 is 'thinking, well, this isn't quite right, and...'. It is the everyday storytelling move for reporting your own inner protest, close to English 'and I'm like, yeah, but'.