Can I make my purchase conditional on the seller serving the tenant an N12 notice before closing?

Yes, you can negotiate a condition requiring the seller to serve the tenant a notice of termination for the purchaser's own use before closing, and buyers who need vacant possession often do exactly this. Your lawyer can help draft the condition to say clearly what needs to happen and by when, rather than leaving it vague.

What this condition cannot do is guarantee the outcome. The Residential Tenancies Act, 2006 imposes strict requirements around this kind of notice, including a mandatory notice period before the termination date and a compensation obligation owed to the tenant, and the tenant retains the right to dispute the notice at the Landlord and Tenant Board. Serving the notice is only the first step; actually achieving vacant possession by a specific closing date depends on the tenant's response and, if disputed, on a process that does not run on the buyer's timeline. Because of this, the condition is best understood as an exit if vacant possession is not actually achieved, rather than a guarantee that it will be. Work with your lawyer on realistic timing given how far off your closing date actually is.