Roganstown charge €25 for opens which I think is a bit steep. It would be better to fill the timesheet at €20 a pop than one third full at €25
Without an established reputation and superb course and facilities price is the deciding factor when selecting an open day. As such an increase in price without major redevelopment or improvements to the course will not to down well with the traveling golfer - especially with the increase in fuel costs which are an ever increasing factor.
However another important factor is selecting a suitable day of the week where there isn't too much competition from nearby courses. You mention Headfort and The Slieve Russell as good courses nearby offering open days. The Slieve Russell have their open days on Mon for €30 and Headfort New on Tues for €25, another quality course in the vicinity is Concra Wood which has an open day on Tues for €35.
I see on the Farnham Estate website that the open days are held on Wed so at least there's no immediate clash there. However Thur and Fri are generally quite days on the open day front so perhaps a change of day could also increase interest? With standard green fees listed at €40 I think it's important to keep the open day price at a half price €20 since a busy time sheet also results in more footfall through the proshop and bar/restaurant where golfers are equally likely to spend much needed funds.