Delighted to hear of this!
If modelling a line which would have been operated like the Achill branch, it's a diet entirely of 4.4.0 D16s 1903-1925, an a mix of these and "foreign" 4.4.0s after that. The odd G2 2.4.0 crept in there too. In its very earliest period (1895-1901) the G2 2.4.0s (MGWR "D" class) and also their "E" class (GSR J26) 0.6.0T.
The Clifden line probably had the least variety of motive power of any line in steam days; in early days the 2.4.0s, but quite quickly replaced by J18 0.6.0s, which would be the sole power on the line after that.
However, imagine this; your line is actually a GSWR one, from maybe Tuam or ballyglunin, heading through Headford and Cong to the other end of the Maam Valley. That gives you an excuse to use GSWR stock, which was more varied and plentiful!
Bearing in mind Galteemore's comments about four-wheeled locos and their layout-wandering quirks, and the availability of the Hattons 6-wheelers (which are close to a GSWR design) and 00 Works J15s, as well as Provincial Leslie's GSWR brake vans, perhaps a scenario like this might be an idea?
As it happens, this is precisely what's behind my own Dugort Harbour. There is a real Dugort - it's a small village on the north side of Achill Island, and at one stage there was a vague proposal to extend the Achill branch across Achill Sound (on what would have been quite a spectacular bridge over a narrow bit of sea containing a quite vicious sea current). It was my original intention that the layout was to be based on this extension as it might have been.
With no JM Design 2.4.0 G2 as yet on the scene at that time (as I would have bought 3 or 4!) but various British 0.6.0s available which I could butcher into a vague equivalent of a J15, I thought I'd switch it to West Kerry, wher you'd lines like Valentia and Kenmare and Newmarket which rarely saw anything but J15s. I had also considered a Tuam - Headford or Tuam - Cong branch. Had such a thing been built, it would have been originall WLWR, then GSWR.
So several interesting scenarios there. I'm a great fan of both detailed models of actual locations and "might-have-beens".
Wandering off-topic, a thing I am more adept at than actually doing anything about it, two other "might-have-beens" which i had seriously considered were both based on the Achill line; in one, it survives into the diesel era with B141s on all trains; in another it was nrrow gauge, survives to the modern era, and becomes a happoy stamping ground for F class diesels, a transferred Dingle 6T after the C&L closed, Walker railcars and so on..........