Clearly there seems to be a link between the two heirs of the House of Usher and the house itself, which is presented symbolically. The "mansion of gloom" is described by the narrator in terms that make it clear that it is rotting and disintegrating:
Its principal feature seemed to be that of an excessive antiquity. The discolouration of ages had been great. Minute fungi overspread the whole exterior, hanging in a fine tangled web-work from the eaves... there was much that reminded me of the specious totality of old wood-work which has rotted for long years in some neglected vault, with no disturbance from the breath of the external air.
This clearly creates a symbolic link between the rotting house and the "rotting" characters of Roderick and Madeline, who are both described in terms that make them appear half-dead and half-alive:
The now ghastly pallor of the skin, and the now miraculous lustre of the eye, above all things startled and even awed me. The silken hair, too, had been suffered to grow all unheeded, and as, in its wild gossamer texture, it floated rather than fell about the face, I could not, even with effort, connect its Arabesque expression with any idea of simple humanity.
This symbolism is perhaps confirmed when, at the end of the tale, the "fall of the House of Usher" is complete in every sense: both of its heirs have died and the house itself is destroyed, cementing the link that ties them together.
Considering irony, you might want to think of the essential ambiguity that makes the "death" of Madeline so intriguing. We are told that she suffers from an illness that has as its symptom "affections of a partially cataleptical character." When she "dies," the narrator comments that the corpse had on its face "the faint blush" and the "lingering smile," but then he explains it away as being natural for a woman suffering from her illness. This helps create the central irony of the text: Madeline is not actually dead, but merely suffering from a bout of her illness. Whether Roderick knows this or not is left undecided. However, even being entombed is not enough to keep Madeline from joining her brother one last time in a terrible act of union that ushers in the destruction of the entire House of Usher.