In 2006 the LIRR conducted an Origin & Destination survey, which aimed to pinpoint exact ridership numbers for each individual station at each time of day. It's a wonderful resource that gives you a good look at LIRR's impressive ridership numbers (you can use the zoom buttons at the bottom to zoom in on the document):
I've used this information in the past when citing ridership at certain stations and when attempting to illustrate service patterns along a particular branch. These numbers were represented in the Ridership Maps I created for each branch.
If you'd like more information about this survey, this document in particular, or other ridership related matters, please Contact Me individually using the form linked.