I'm not up to the part about the ride yet (almost), but the meeting of Irene and Soames seems to explain much of their relationship. Irene is living with her widowed stepmother, has no money, and is now an impediment to Mrs. Heron's remarriage. I'm sure she had pressure from all directions as Soames pursued his objective. Irene's reaction:

And she had shuddered--to this day he had not forgotten that shudder--nor the look so passionately averse she had given him.

Finally, a year later, Irene succumbed to the pressure. I think it is quite safe to imagine that she wanted no part of the marriage in the first place, and she must have been ultimately forced to the union by circumstances beyond her control.

The difference between Irene and Maria Bertram Rushworth is that Maria had access to money in her own right, pursued the Rushworth relationship out of desire for wealth and consequence, continued to pursue the marriage to an unsuitable partner despite being in love with another and Sir Thomas giving her a chance to back out of the arrangement (admittedly Sir Thomas was not wholehearted in the offer). I could go on, but this isn't the MP board, so I won't. ;-)

It was a difficult state of affairs for women who had no other real options than to rely on marriage for their living. After reading about the so-called courtship of Soames and Irene, it really gave a different spin on the concluding sentence in the prior chapter: And moved by some inexplicable desire to assert his proprietorship, he rose from his chair and planted a kiss on his wife's shoulder. Very sad!

Back to reading. I get the sense some of these characters are going to driven to desperate acts...