It's hard to make a blanket statement on this; there were about 8 million people in Ireland at the time of the famine, and it's likely that some of them, at least, simply weren't aware of which plants could safely be eaten and which not. There are certainly folk tales of people being found dead by roadsides with grass stains around their mouths.

However, I'd be inclined to treat the statement - and the folk tales - with a considerable amount of scepticism. The Potato famines of the mid-1800s were devastating because of a cascade of events. The potato is very well-suited to the Irish climate. It grows where almost nothing else except grass and heather will, and some of the varieties grown in the 1800s had an incredible return - the main one being the Lumper, now known as the White Lumper, which could often return 20, 30, or 40 tubers for each one planted. They were watery and didn't taste great, but there were a lot of them. So, of course, the people of Ireland planted them, and ate them, and planted more, and had children, and their children planted and ate potatoes, and the population boomed to a degree that couldn't possibly have been sustained by other crops.

However, the potato wasn't all people ate: they also had milk from cattle (and sometimes from goats) along with butter and cheese, they had fish, and they had other vegetables (cabbage and turnips) and a wide array of foraged and semi-wild produce. There is a lot you can eat in the Irish countryside, and the poorer folk depended on foraged food to supplement their diets. They even ate some bread and probably a few varieties of porridge, which had been the staples of the medieval era.

The issue in the famine wasn't the lack of availability of those other foods, per se; it was the lack of the potato. When the blight hit, in 1845, it took out not all, but most of the potato crops, fairly randomly. They rotted in the fields. This meant that the core of the people's diet was gone. They could get along for a bit on the foraging and other foods, particularly in autumn, when wild food is pretty abundant. The real issues, the worst of them, didn't arise until around February to March 1846, when what potatoes there were had run out. They could eat the seed potatoes, sure, but then there would be nothing to plant in late March and early April. And crucially, this is the spot in the year where there is very little wild food to be had; it's called the "hungry gap". No matter how much you know about what's edible, that doesn't serve you when there's nothing there but grass and and heather and evergreen trees and the stumps of ferns.

So some folk ate seed potatoes and some killed and ate their cattle (and chickens, and horse, and, it's rumoured, sometimes dogs). But those folk then had to buy or beg for seed potatoes for small crops the following year, and didn't have the cow for milk, the chicken for eggs, or the horse to draw the plough. And the blight struck again in August 1846, and was rather more widespread, and the whole thing got much worse.

So, overall, no, the Irish had not lost the ability to identify wild food, and they used it as best they could to survive through the Famine. But when you have what's essentially a monocrop supporting a huge population, and it fails, no knowledge of wild food will save everyone.

Sources:
A.T. Lucas: Irish Food Before The Potato (Gwerin: A Half-Yearly Journal of Folk Life, Volume 3, Issue 2, 1960)
The Great Irish Famine, Cormac Ó Grada
This Great Calamity: The Great Irish Famine: The Irish Famine 1845-52, Christine Kinealy
Potato Varieties of Historical Interest in Ireland, Department of Agriculture, Fisheries and Food