Express your unconditional love for them.
Try to separate the frustration or bother from the person.
If someone does something irritating to someone else, it is not the person, but the action that is irritating.
Also, understand that we each create our feelings. If we are irritated, we create the feeling -- we are reacting to the stimulus.
Forgiveness can only truly happen when the person takes 100% responsibility (leaving no room for blame), faith (100% confidence) that all will be well, 100% unconditional love, and 100% humility (generosity and self-less-ness).