In A Doll's House, why is it ironic that Nora helps Mrs. Linde gain a position at the bank?
In helping Mrs. Linde secure a position at the bank, Nora is putting herself in a dangerous position. This is ironic in the same way her ill-begotten loan is ironic. In both cases Nora is attempting only to help someone and ends up putting herself in a dangerous spot.
Practically, Nora accidentally gets Krogstad fired when she asks Torvald to help Mrs. Linde get a job at the bank. The result of this turn of events leads Krogstad to visit the Helmer household, confronting both Torvald and Nora. Until Krogstad is fired he poses little danger to Nora but when he loses his position (along with his chance to rebuild his public reputation), Krogstad begins to threaten to reveal Nora's secret loan and her crime of forgery.
Another irony can be seen here. Krogstad is fired from his position at the bank for two reasons, one of these being his own past crime of forgery (for which he escaped punishment). An example of situational irony, this parallel between Nora and Krogstad builds upon a theme of punishment and the search for redemption as well as a theme related to the costs of dishonesty.
The conflict between Krogstad and Nora, sparked by Krogstad's firing at the bank, leads Nora in the end to follow the same path of self-searching and redemption that Krogstad follows in the play.
Krogstad is desperate, so initially, he appears to be a villain; in fact, he has been trying to remake his life after having made earlier mistakes.
We see once more another layer of irony as Nora figuratively switches places with Krogstad as a result of Mrs. Linde literarlly taking Krogstad's place at the bank.