I searched the Irish Family History Foundation's database. It contains 11,500,000 surviving rcds for abt 60% of Irish counties. Rcds B4 1840 are sparse. Recording of B/M/D events came rather late to Leitrim.
I searched for a marriage for the surname Costello prior to 1830. Only 5 of 24 parishes/districts have rcds in this era. I found 0 M rcds for William Costello (+variants).
There is 1 B rcd for Joseph between 1830 and 1849. This rcd is in the RC parish of Mohill.
There are 14 B rcds for Michael ditto. Three are 3 rcds in Mohill: 1843, 1844, & 1849.
There are 8 B rcds for Thomas ditto. There are 0 rcds in Mohill.
There are 4 B rcds for James ditto. There are 0 rcds in Mohill.
There are 2 B rcds for William ditto. There are 0 rcds in Mohill.
There are no B rcds for Owen Costello prior to 1852.
As you can see, I was trying to find a common birth parish for the forenames. It didn't work. As 10 of 24 parishes did not begin rcdng Bs until after 1850, there is a high probability that the B events were not rcd'd, or rcds did not survive.
In my searches above, I used your stated ranges. You may want to modify these assumptions. Suggest you google the Foundation's name, register, opt to be notified as rcds are added, and browse those that have. Registration and browsing are free.