Look for a marriage on:
Lancashire BMD
www.lancashirebmd.org.uk You can also search index for the children's birth registrations on Lancs.BMD. Unfortunately mother's maiden names haven't yet been added to index of this or GRO for a lot of Manchester births. Lancs. BMD shows sub district, so more local than GRO Index.
Lancashire Online Parish Clerks
www.lan-opc.org.uk This is in 2 parts - lists of churches in each area with transcribed registers for some churches, mostly C. of E., with some R.C. and Non-Conformist; and Ancestor Search section. Search options in Ancestor Search are individual church, place or whole county.
Lancs. BMD and LAN OPC are both volunteer-run sites. They are constantly being added to, so if you don't find something at 1st go, look again in a few weeeks, months, years.
As the people you're researching were in Manchester have you looked at Manchester & Lancashire Family History Society? The Irish group isn't operational at present. The website has a list of churches with dates of registers and locations of microfilm and transcriptions. It also has a list of cemeteries. A M&LFHS project is a searchable index by name of Catholic church registers in Manchester. I don't know churches or years covered or if it's finished. Website has guides to Manchester FH research.
www.mlfhs.org.ukGEN UKI has information about places including lists of churches with dates and maps. If you know where a person was living at a certain time do a distance search for churches of the relevant religion in the area, check if a church existed at the time, find it on the accompanying map and make a shortlist of churches they may have been baptised or married at. Church info includes whether it has/had a graveyard and when. Google GEN UKI then look up a place. It's another volunteer site. Some places have better coverage than others.
If your ancestors were Catholic take a look at Catholic Family History Society. Catholic FH research in England has particular difficulties. There are some informative articles on this website. Catholic burials in 19thC Manchester for instance.
Edit. I missed off the word "registers" from "A M&LFHS project is a searchable index by name of Catholic church ..." You can enter a surname (+1st name + date if known).