To unignore them with a command: /ignore -r nick [mask number]
Or, to use mIRC's built-in dialog interface:
[*]Tools
[*]Address... (Alt-B)
[*]Control tab
[*]Select Ignore from the drop-down combo
[*]Select (left-click) the address you wish to remove
[*]Click the Delete button
[*]Click the OK button