Can we break a rental lease because there's a homeless camp outside?

Because you've discovered you don't like the neighbourhood? No.

I don't think it matters whether there's a high or low level of homelessness in the area. The landlord's obligation is to afford the tenant quiet possession of the premises. The landlord has no more control over what goes on in the neighbourhood around the premises than the tenant does, and is not responsible for it. Prospective tenants generally inspect a premises before renting it, and do their research. Part of that process involves making a decision about whether the neighbourhood is a neighbourhood in which they wish to live.

[It might be different if the landlord made some express representation about the neighbourhood to induce the tenant to enter into the letting that turned out to be false. ("The roads and services are in charge of the local authority! There's an excellent school five minutes' walk away! A fast bus to the centre of town passes the end of the road every ten minutes!") That might give some grounds for seeking a remedy including, if the misrepresentation was fundamental enough, termination of the lease. But there's nothing like that here.]

I very much doubt that you can do anything at all unless the homeless person in question is living on the premises. The landlord can't control what happens in the surrounding area. Homelessness is not something that will in improve in the years to come, unfortunately.

If you really want to leave, offer to the LL that you reassign the tenancy i.e. you get someone to take over your tenancy agreement (Subletting and Assignment | Residential Tenancies Board (rtb.ie)).

If the LL refuses, you can break the lease: Tenants ending a fixed term tenancy | Residential Tenancies Board (rtb.ie)

It's very hard to find a place though so maybe give it a while and see

I would get out if I was you even at a cost to you. This is not how you want to live. Still ask for your deposit back. Tell landlord your wife doesn't feel safe.

Giving the bushes a serious haircut will likely encourage the current inhabitants to look for a new more secluded place to hang out.

I never check for people living in the bushes where i intend to move to be fair. They would be hard to spot unless you are there when they come out of the bushes.

Though Malahide beach is starting to look like Venice beach now.

Are you not too busy breaking up with your missus as per this morning?

Don't create any further threads here until you've learned not to troll.