When I cast my mind back through the plays I've seen, different memories offer themselves: electric moments that still seem vivid but soften at the edges when I try to fill in the picture; several takes on the same plays, conventional and radical, that interweave in a strange mesh; a myriad imperfectly recalled Brian Friel quotes that I need to Google.

The wonderful thing about the theatre, it’s liveness, is also its torment: it’s irretrievable.

Anyone over the age of 30 can probably relate to the entropy of experience over time, where details distend, fade and finally slip away completely. Billy Collins's poem Forgetfulness puts the matter in (hopefully) memorable terms: "The name of the author is the first to go/Followed obediently by the title, the plot,/The heartbreaking conclusion, the entire novel."

With a novel, at least, you can go back to the start. But all those cherished memories of Donal McCann in Faith Healer, Siobhan McKenna in Bailegangaire, and Cillian Murphy and Eileen Walsh in Disco Pigs will begin to stagger until they are long past jogging.

The crueller realisation is that theatre doesn’t just reside finally in our bockety brains – it requires a serviceable memory to happen at all. There are few things more chilling than the sight of an actor drying onstage, missing a cue, fluffing a line or requiring a prompt, because it nags at the fragility of all memory and the vulnerability of a profession.

There's something poignant, then, about the fact that Angela Lansbury, pushing 90, and Michael Gambon, who has worried routinely about Alzheimer's, have both resorted to using an earpiece to feed them prompts.

It’s a divisive issue. Some associate professionalism with a mastery of the script – that if an actor is truly inhabiting a part, they have first learnt it off. Others insist it is no different or more compromising than having a prompter backstage with the script. But actors have been fired for failing to remember their lines (and one American actress who relied on an earpiece in the 1980s used to regularly pick up taxi signals).

Given that new plays, especially, may go through successive drafts and last-minute edits right up to opening night, and that those all-prevalent monologue plays give an actor no supporting cues to lean on, shouldn’t we make allowances for the occasional lapse? Besides, it will all be forgotten in time.

Memory, however, is a major concern of theatre. Lapse, a new show from the magician and mentalist Shane Gillen and Sugarglass Theatre, explores the distortions of personal recall, scientific studies into repressing trauma and the mnemonic methods of illusion.

Friel's Aristocrats, perhaps his most Chekhovian play, is at the Abbey, where a family prop up a past that is not quite in accordance with reality. ("Remembrance of things past," wrote Proust, "is not necessarily the remembrance of things as they were.") And The Price, at the Gate, is all about the inheritance and dispute of painful memories following the death of a parent. Incidentally, it's about 10 years since the Abbey and Gate staged those plays – waiting, presumably, for recent memories to fade.

Perhaps it’s the instability of memory that makes it possible, even necessary, to revisit the theatre, to buttress our experience and replenish our store of souvenirs.

As another Friel quote goes, which I do remember clearly: “To remember everything is a form of madness.”