Elliott family of Milltown Co Cavan

The only possible lead I can see is a baptism of what might be a sibling to your William - a John Elliot baptised on the 26th December 1851 in Drumlane Catholic Parish, County Cavan / Fermanagh. The parents are recorded as James Elliot and Jane (no maiden surname noted). Drumlane parish was sometimes known as Staghall or 'Staghall and Milltown' after the names of the chapels for the parish, one at Staghall the other at Milltown.

The baptism registers for the parish go back to the 1830s and are free to view on the National Library of Ireland website - there are no significant gaps noted, but it's possible there are faded, damaged or missing pages. It may be worth a closer look through the register images to see if baptisms of other possible children can be located.