Is there any documents I can research to find when Patrick Mulholland and Bridget Henderson came from Ireland?
If you mean are there any passenger lists, the answer is no. Ireland was part of the United Kingdom, and no-one kept records of internal movements of people. The only way you can trace roughly when they came to Scotland is if you find them in a census with older children born in Ireland and younger ones in Scotland, or if they ever applied for poor relief, in which case the poor relief records will almost certainly say when they arrived.
Can their fathers be traced as all it says is "Ireland" on the certificates?
That depends.
I see that Patrick Mulholland and Bridget Henderson were married in Glasgow in 1876. If the marriage certificate you have doesn't say who their mothers were as well as their fathers, you have not got an authentic certificate and you have not got all the available information. Go to
www.scotlandspeople.gov.uk, buy a few credits and view the original certificate. If/once you have got a proper certificate, tell us everything that it says.
If Patrick's and/or Sarah's parents also moved to Scotland, and if they died in Scotland, you should be able to find their death certificates, and if you can find them, they should tell you the names of all their parents including their mothers' maiden surnames. So you need to look in the census before 1876 to see if Patrick and/or Sarah were in Scotland and living with their parents before they were married.
Finding records in Ireland is likely to be difficult. To marry in Scotland in 1876, Bridget must have been born before 1854, and Patrick before 1852. You presumably know how old they were because you say you have the census. Tell us everything the censuses say.
The marriage certificate will also tell you whether they were Roman Catholic or Protestant. This can make quite a difference if you are looking for records in Ireland. Protestant marriages were (supposed to be) registered from 1845-ish, but Roman Catholic ones were not registered by the civil authorities until 1864-ish. You can view indexes to, and some originals of, many Irish births, marriages and deaths at
https://www.irishgenealogy.ie/en/Once you have told us everything you already know (and I do mean
everything - you would be surprised how often an apparently minor detail makes all the difference), it would probably be worth moving this thread to the Ireland forum.