The difference between a phone call and other distractions such as eating, smoking, adjusting radios, heating, or even changing gear is that for many people holding a phone conversation ends up taking almost 100% of their concentration. (I'd guess introverts will find phone calls more demanding than extroverts.)
The other stuff is done either unconsciously or when you know the road ahead is safe for a second.
A phone is different from talking to a passenger as most passengers see road dangers and know when to stop talking.
I would guess that a large proportion of minor collisions around cities and towns are being caused by mobiles or at least distractions of some kind. But we may never know the stats since the person who causes a minor accident isn't often going to admit they were on the phone, and even if the gardai are called they're not going to make any serious attempt to check phones if no-one is injured.
Moving on to higher speed roads where more fatal accidents occur, I'd guess phones are less of a cause of accidents, these roads are probably more forgiving (in terms of number of accidents - not severity) of distracted drivers. The accidents may be caused by bad luck, or impatient, inexperienced, aggressive, drunk, ill drivers instead.
As for gardai stopping drivers, the problem is that most of the time you see someone you're sure is using a phone you're driving past them the other direction, it's the same for gardai so pulling them over usually isn't straightforward. Maybe if they've a dash-cam they could issue fines like they do with speed cameras but they'd still need to be able to see the license plate.