Thankfully Nessa hasn't had any colic type issues. She's been easy to console, mostly just wants to be held.
However, when Ronan was little, he used to scream bloody murder from about 5pm-10pm, until he'd finally pass out from exhaustion. Nothing would console him. We talked to our Dr. about it, and she suspected that he might be getting over-stimulated. Basically he didn't quite understand what it feels like to be tired and what it means, so he would freak out and cry. We'd try to console him, which was just more stimulation, so he'd freak out more.
She suggested that we pick a time for bedtime and make a routine (put on his jammies, place him in his bassinet with the room darkened, turn on a little music (we had a mobile that timed out after 5 minutes for this), say goodnights), then let him be for 15 minutes. If he was still crying after 15 minutes, we'd check on him and make sure he's ok (fed, diaper changed, etc) and start over until he fell asleep.
The first night, he cried for 1 1/2 hours instead of 4. The next night he cried for 30 minutes, the 3rd night for 5 minutes. After that he would fall right asleep when we'd put him to bed, and was an awesome sleeper from then on.
I know not everyone is a fan of CIO of any kind, but this did really help us, so I thought I'd pass it on.