I've been searching the subscription site Roots Ireland and so far have not identified any good leads in either Co. Roscommon or Co. Leitrim. It would appear that the church where the children were baptized no longer has records back to the 1840s/1850s. No marriage record either.
Do you know for sure when they left Ireland?
Civil registration of births started in 1864.
There are five Griffiths Valuation head of household listings from the mid 1850s for a Thomas Mulvey (3 in Roscommon and 2 in Leitrim). We don't know for sure if Thomas was leasing a large enough plot to be captured in the Griffiths.