D250,
well spotted, I had never considered that RA109 actually replaced another RA, but I think it did.
I found a record of the first RAs to arrive in Clontarf. They were RA 35 and 37, which I recorded at Clontarf on 28/4/76, just 30 years ago. By 1/5/76, RA33 - 38 were there. These were followed by RA 27 and 28 (by 22/5/76) and within a few dayslater RA 26, 30 and 32. Note RA36 is included, but it was not in the continuing fleet. So it looks like RA109 was sent to Clontarf to replace RA36. I have RA109 recorded as in Clontarf in June 1976, so the replacement must have happened very quickly.