If you reach this point, about a mile and a half into the hike, you've navigated several forks successfully, and once you're past this one you can hike for several miles without having to make any decisions at all. Remember that either "John Muir Route" or "Donohue Pass" are indicators that you're headed the right way, and then you can proceed with confidence. The wrong fork at this intersection will start you on a long uphill trek to Tuolumne Pass and beyond.
Ireland Lake is not on the Lyell Canyon trail. To get there, you'll have to take another fork four miles (6.5 km) ahead and then hike steadily uphill, including yet another fork, for several miles after that.