In "time" terms, possible. The 121s appeared in 1961 and the very last six-wheelers were withdrawn in early 1963.
However, there were only so many 121s, and they were actually used a lot on freight in their first year or two; passenger usage was all main line. None ever worked regularly on branches, and six-wheelers were by then only to be found on the Ballinrobe, Loughrea, ballaghadarreen and Kenmare branches, all of which were fully steam worked until after the last six wheelers were withdrawn.
However, every good rule has an exception. Cork retained a couple of rakes of the six wheelers right to the end, and they were retained for use on Youghal excursions. It is therefore at least theoretically possible that a 121 hauled one of these - it would have to have only been in the summer of 1962. I'd almost lay money on it that they didn't, but they COULD have - and as always, "Rule 1" applies! I've an 00 Works CBSCR saddle tank on Dugort Harbour until the end of steam in '63, some sixteen years after the last of the real thing was scrapped!
There's another exception. Several ex-GSWR FULL passenger brakes wre in use until 1968-70, and these are the inspiration for the Hattons black'n'tan brakes. THESE certainly WERE hauled by 121s, in particular on the galway mail trains - but in black'n'tan days.
Correct - a 121 hauling anything in the darker green livery wouldn't have happened - it was not to be seen on main lines by the time 121s appeared. Lighter green in the circumstances described above.
Of course, when 121s first appeared in grey, ALL passenger stock, including the most modern laminates, were green, or else the short-lived "silver". The earliest vehicles in black and tan started to appear when the 121s were some 18 months old.