I research the Irishgenealogy.ie free, online database et al. I found NO M rcds for your named ancestors in this database. I confirmed that the Dennehys occur predominantly in Co. Cork by visiting a different database.
For Jeremiah Dennehy M ????, Co. Cork, I found 27 rcds beginning in 1778.
For Elizabeth Cahill M ditto, I found 15 rcds dating from 1797.
For Thomas Dennehy M ditto, I found 28 rcds dating from 1803.
For Ellen Jones ditto, I found 18 rcds.
It is possible to further refine the above for each set of marriages to find the yr and parish sets for the bride and groom. These sets can then be compared. If you find matching elements in each set, it is likely these, if any, are the records of these ancestors. If NO matching elements are found, this infers the rcds didn't survive, or haven't been transcribed. Contact me by e-mail if you have questions.