GNU less: How can I search while ignoring case sensitivity without using less -I option?

Solution 1:

You can set it from within less by typing -i and then doing the normal search procedure.
Have a look in the help for less by pressing h

Solution 2:

After turn on Ignore case in searches by pressing -i you have to put string in lower case. If your string has upper case chars your searching will fail (find only strings with upper chars)