Hello ~
My father just found these posts after I announced I was finished with out "Merrigan" branch. I've been doing research on this branch for roughly about 30+ years. I have lots of info and connections. Plus still always looking to connect with others with reliable information. So this is the info I have at the moment re: John Merrigan , and yes you're right going further back will take an act of congress or to find an old family bible ~ Hopefully this helps

John Merrigan b. abt 1793 Blessington, Wicklow, Ireland / d. abt 1875 Bolton, Lancashire, England
m. Mary Lynch ~ b. 1795 Co. Wicklow, Ireland / d. New Ross, Waterford, Ireland ~ She was previously married to a gentlemen by the last name of RILEY.
children:
Bridget Merrigan
Eliza Merrigan
Margaret Merrigan
Thomas Merrigan
John Merrigan b. 1817
James Merrigan b. 1821
I'm confirming some other information but now that I've found your posts I'll be able to reconnect once that information becomes available.
Nice meeting new relations. Welcome aboard!
Tara-Leigh Merrigan (Doyle)