I'm so sure that this must exist, but if it doesn't maybe there is a macro for it...
One of my most favourite features of vim is the insert before when in visual mode (<C-v>, select the lines, <C-I>, type a little, then Esc).
My issue is that I want to paste the clipboard contents before, not 'insert'. I tried <C-P> but no love.
I have read cheat sheets, searched everywhere, looked through the questions on here and haven't found it, but I could definitely be searching for the wrong thing.
I assume you mean using I in visual block mode to insert the same text on multiple lines, where hitting p simply pastes on the current line rather than all the selected lines.
In insert mode, you can hit C-r followed by a register to insert the contents of that register, so if you wanted to paste the unnamed buffer, you'd enter
C-r"
Similarly, to paste from the clipboard
C-r*
By entering insert as you normally would, then using C-r, you'll get the text on all of the selected lines.
Take a look at :h registers to see what registers are available to you.