Ironwood doesn't think it was any one point in time that made him realize he loved you.
He felt this warmth in his chest whenever you'd teased him when you were young even if he tried to keep himself stoic and straight-faced; you were one of the only people who managed to conjure a smile from him when he wanted to do anything but. You were always honest with him, offering up your thought-out opinion, letting him know if you thought he was injecting his emotions too much into a situation and not viewing it as objectively as he should.
You were the balance he didn't always appreciate but who he always needed.
He hadn't been the first one to say 'I love you', you had beat him to that punch and you had joked that you always knew you would. It wasn't a matter of not feeling that way, his feelings for you had been strong since day one, but it was a matter of not feeling worthy of those feelings. Did he deserve to love you? Did he deserve to have someone like you in his life loving him the way you did? He wouldn't say he was a man of low self-worth but there was something about you that had him questioning his own motives, his own needs, and whether they were genuine enough that his connection with you was real.
But at night when he slept alone without you by his side, when you were out doing your best for the sake of the world, he missed you. He felt that dull ache in his chest as if something was off; he knew you didn't belong at his side as you were a free spirit who preferred to spread your influence around rather than stay in Atlas but it still felt wrong not to be with you. Especially after all communications are down he finds his thoughts filled with you wondering what you were doing, if you were okay, when he'd get to see you again...
Was this what it felt like to love someone?
He thought so.