Islands off Scotland west coast vs Ireland’s west coast.

Yes. Apart from anything else there are many, many more islands off the coast of the UK than off the coast of Ireland. (C. 4400 vs 80).

Is there much difference to warrant visiting both country’s western coastline?

They are very different - both geologically (which affects scenery) and culturally.

There are a lot more to explore off the coast of Scotland - to do them justice would take months!

Getting to the Irish islands is a lot more difficult because you have to do them individually, going to one port, to that one island, back to the same port, drive to the next one and repeat etc. They are widely scattered and most of the ferries have pretty limited timetables, with exceptions.
In Scotland you can use CalMac to hop between all the inhabited islands, so that you are progressing in a reasonably sequential manner without keep on returning to the mainland. But even then it would take you several weeks. And although the timetables are good and keep improving they are still relatively limited.
It's easy enough to stitch such an itinerary together but certainly weeks are needed.
'Not much time' simply won't work.
And you don't want to just port hop, you want to assimilate each island. Even in a group (like The Small Isles or the Slate Isles) each is very different.
The long chain of the Outer Hebrides changes radically from the deeply Roman Catholic south of Barra to the strong Protestant influence of the 'wee frees' on Lewis. You need to take that in, understand the culture, not take it as a quick tick list to be done in the shortest possible time. You get nothing out of that.
And the furthest island St Kilda (now uninhabited, but an iconic place to visit) can take days of waiting for a weather window.
You aren't even talking here about the Northern Isles of Orkney and Shetland, or the isles of the East Coast of Scotland.

Without specifying exactly how much time you have (and what are your interests in possibly going there), it's impossible to answer the OP's questions in any meaningful way.

For starters, having "not much time" is pretty much a non-starter for either. Everything depends on where else you will be (coming from/going to) and how you define "not much time" in this case.

Thanks all. Now I know I must find time for both countries west coast. Will skip Dublin.