What is the ironic device in this quote John Proctor says, I have gone tiptoe in this house all seven month since she is gone?John says this...

This line occurs in a particularly uncomfortable scene in
The Crucible.  John, we've discovered, has had an affair with the
"dissembling" Abigail.  We've also discovered he has no intention of being with her,
since she gave him that opportunity again in Act I and he denies her--something he did
on his own, not because Elizabeth was mad or was somehow aware of their
encounter.

Here, Elizabeth is telling John about the trial
(they'd both been out of touch with the proceedings in town), and John is dumbfounded,
telling her he knows the girls to be frauds.  This opens up the entire episode between
John and Abigail at Betty's bedside.  John apparently told Elizabeth everything about
the encounter--except for the fact that he and Abigail were alone when it
happened. 

Elizabeth simply makes a few statements, but
they're enough to work on the already guilt-ridden John.  The lines which come before
the line you mention is:

readability="6">

You forget nothin' and forgive nothin' Learn
charity, woman.

Then
comes:

I have
gone tiptoe in this house all seven months since she is gone.  I haven't moved from here
to there without I think to please you, and still an everlasting funeral marches round
your heart.

And there's
more. 

The irony of these lines is that Elizabeth has,
indeed, forgiven her husband.  The problem is, hehas not forgiven himself.  This, of
course, is the crux of his dilemma at the end of the play when he must lie and live or
tell the truth and die.