It was suggested that the registration might actually be BI 1552, which would be consistent with Co. Monaghan. The BI series was used from 1903 to 1961, but I don't know if there'd be any information to help to narrow that down at all.
However, this picture is rather small. Is there any chance you could do a bigger copy of it, please - ideally around 600 pixels wide would be good. Some of us are getting on a bit and our eyes need all the help they can get!