Hi Anne, the head constable would I think be just above sub constable so not too senior, yes there were catholics in the police at the time, she got married during or towards the end of the famine.
As I understand it, the police were recommended to join by a local dignitry, landlord or parish priest etc and the letter should exist, place of birth was recorded, possibly only county, time of resignation and were pension was paid, there was a book printed a few years ago on the history of the Royal Irish Constabulary by Jim Herlihy, he is also found on the private Facebook page 1816 to 1922 - Royal Irish Constabulary A Forgotten Irish Police Force, they appear to be very helpful, I have not posted on it but enjoy the information, often they will post pictures of the barracks and what it is now if still standing. The notebooks may stll exist also but are held in Kew in London I think. Jim Herlihy also produced a book on the Irish Revenue Police, disbanded in 1852 and incorporated into the Irish Constabulary, a number of McManus names in it, but if your man was in the constabulary in 1851 he was not in the Revenue Police (raiding illegal alcohol stills appears to have been their main job).
Do not worry about the delays but if you wish to speed up queries my site email is stpeters@irelandxo.com I notice your Old Mohill posting did not garner much response, I am the Paddy who gave it a like but it appears to have gone down the line quickly, a lot of posts appear very quickly. However if you were to ask about the location of the former RIC barracks it will push the query back to the top and you may find where they lived for awhile.
I can photograph the tiny bits from the books for you if you mail me too. Said Lord Leitrim was assassinated in 1876 in Donegal, it is on the site I sent you previously.
Not sure how we can determine if it is your McManus but it is an option, I live about 70 miles from Mohill on the east coast.
Happy Hunting,
Pat