Hi Mike
I'm no expert on Irish buses, but this is what I have been able to find so far.
Bus manufacturers made the chassis, and with a very few exceptions, the bodies were usually built by separate coachbuilding firms who gave the bodies their own 'style.'
The Guy Arab II did run in Belfast. HERE is a link to MZ7421. Belfast also had the later Guy Arab III chassis. I'm not familiar enough with the Farish model to tell if the bodywork is similar to the Belfast version.
The Leyland Atlantean. Arguably Leyland's most successful bus with over 8000 built. As you would expect, there have been many different body styles built on that chassis over the 38 years it was in production.
The Farish model appears to be based on an Eastern Coach Works (ECW) body, with the separate engine bustle at the rear. Similar bodies were available on the big island from Northern Counties and from Alexander. As far as I can tell, all of CIE's Atlanteans were bodied by Van Hool McArdle, who designed a body without the distinctive rear engine bustle. The answer to your question whether an Atlantean matching Farish's body style ran in Ireland I am 99% sure is no, unless anyone else knows differently.
If you want to find details of a specific bus, Bus Lists on the Web is a useful starting point, although not all the photo links from that site to Flickr work.
https://www.buslistsontheweb.co.uk/
Hope this helps.
Moxy