I was curious myself, so I did some searching on the Internet (notice I
didn't use the word "research"), and it appears that
1) Twain could either be refering to Finn McCool, an old Irish warrior
fabled to be the creator of the Giant's Causeway in Northern Ireland, or
Patrick Cotter, the Bristol Giant, who died in 1805 and was later exhumed in
2) "Tilters" is slang for hoopskirts.
3) I think Twain's female companion had fallen on top of him and he had
somehow found his way underneath. The support for those skirts does indeed
look like the degrees of latitude and longitude.
I've included links below to the sites where I obtained the above
If you obtain any further clarification on who the "Irish Giant" is that
Twain is actually refering to, let me know. My OCD-like thirst for useless
information never ceases :)