To copy and paste in mirc (or anywhere else) simply
hold your left mousebutton down while you drag the cursor over the text
that you wish to copy.
Then release the mousebutton, the hilite will disappear.
If it does not, then press your CTRL button, hold it down and press
the letter C and then release both buttons. Your text has now been
copied to your clipboard.
And now to paste the text where you want it place
your cursor where you want the text entered, press your Ctrl button,
hold it down and press the letter V and your text will be pasted.