The old 10" shell gun of 86cwt is the worst possible gun to use as an example. It was notoriously weak and had already been condemned in the 1850s (in fact, in Dahlgren's Shells and Shell-Guns book, the range table for the 10" is crossed out completely). This is what compelled the Navy to purchase Dahlgren's shell guns in the first place, to provide a X-inch replacement. The penetration of the IX-inch gun was rated in period information as 30" at 1300 yards, thus my comment about it almost penetrating a liner at range (though this means the shell explodes in her side anyway). This compares to 16 inches for the American 8"/63 and 21 inches for the 32-pdr/56cwt at that range. The IX-inch will comfortably penetrate the sides of a frigate like the Shannon. If the Liffey even gets a rifle, it would have been in place of the 68-pdr anyway, the only important gun on the ship. Otherwise it's the same disparity in firepower you'd see in an 18-pdr frigate taking on a 24-pdr frigate in the old days. Each individual deck plus the pivots is capable of firing larger shells further than on the British ship. It's the literal one the American ship was specifically designed to defeat.Unfortunately by 1861 the average RN frigate is getting some rifles in and hence has been upgraded.
It's not correct to say a IX inch Dahlgren would "still easily penetrate" the side of a frigate and possibly a liner, because it looks like the penetration of a liner can't really be predicted from that gun. Liners are planked to 30" or even 36", while the Colorado was planked to 24", meaning a long 32 pounder will penetrate the Colorado at 1000 yards with shot, but not with shell, and the 8" will nearly penetrate with shell; however, the liner will only be penetrated by the 64 pounder with shot (and nearly by the 42 pounder if the liner's only planked to 30".)
The 64 pounder's penetration is achieved with shot while burning 16 lbs of powder. Since the IX Dahlgren only had a 10 lbs powder charge approved (smaller charge, heavier projectile, hence lower speed and lower penetration) it seems it would not be able to penetrate a liner's sidewalls with shot let alone with shell. The IX Dahlgren is actually burning less powder than the 42 pounder.
The penetration of the 10" old shell gun with 10 lbs of powder was about 24" oak when firing shell. Based on that and the powder load of the IX inch, we can say penetration of an enemy heavy frigate would be "possible but marginal" - a liner, however, is right out.