Yes, I think I'm just going with Occam's Razor on this one and thinking that the phone call was either mistaken timing or mistaken identity and they had an accident/health incident and drove into the river. It answers all the questions about where their car is, where they might be and why they left without taking anything other than immediately required items.
I could be wrong of course...!
I think this highly likely too, but with the same proviso (and - unless I'm missing something obvious - I'm still not at all sure why the phone call is a factor). To hijack the old 'butler did it' cliche, I have to say that when an apparent mystery like this occurs near a body of water, then 'the water did it' is probably not a bad bet.
Seems that Fermoy is prone to really quite serious and regular flooding. Looking at online reports, the main flood season appears to be at the tail end of the year, but I don't suppose that precludes less severe flooding at other times of year - although I couldn't find a record of anything in April/May 1991.
Here's an image of Fermoy showing what I make out to be the approximate location - at least as reported in the previously linked article - of the car and body in regard to the
earlier missing persons case that was - prior to the discovery of the vehicle and body - being linked by some to the
later Dwyer incident:
Chapel Hill - where the Dwyers are reported to have lived - is located just to the right of the spire in the top left corner: it's quite a steep road, stretching towards the river, but not all the way to it.
The flooding can be really quite impressive:
As I've suggested, although the above images are from later in the year than either disappearance occurred - you can see how a relatively minor event - compared to these - might still be considered quite hazardous.
That said, this could be a complete red herring - although an albeit not exhaustive bumble around on streetview doesn't provide an obvious entry point for regular traffic in normal conditions. (But then, as already admitted, the idea the Dwyers entered the water at all could be a similar red herring.)
Ashe Quay (hard by the river, far side of the bridge, left hand side - in the first image) seems an option at first - but there appear to be obstacles in the way of an easy entry. Possibly these did not exist at the time (the work their now does appear quite new - but that's not to say there wasn't something similar before). I did wonder, in the earlier case, if the victim had been a fisherman, and had maybe driven through fields on the north side of the river (right hand side in first image) and misjudged the bank - but that's just me freeforming. If he'd been an avid angler, I dare say it would be mentioned somewhere.