In Irish, if you are look to have:

Let's see

or

Let's do

or

Let's say.

How do you construct that?

1st person plural of imperative mood:
Feicimis, déanaimís, abraimis in Standard Irish.

Lars

Lars is right, grammatically. In everyday speech depending on the context / Ag brath ar an gcomhthéacs / one might hear:
Let's see
Lig dúinn féachaint
Fan go bhfeice muid

Let's do (it)
Tugaimis faoi

Let's say (that the Taoiseach resigned)[= Let us suppose?]
Cuiream i gcás (gur éirigh an Taoiseach as oifig)

The third verse of "Óró, 'sé do bheatha abhaile" has two of these:
A bhuí le Dia na bhFeart go bhfeiceam
Muna mbeam beo ina dhéidh ach seachtain
Gráinne Mhaol agus míle gaiscíoch
Ag fógairt fáin ar Ghallaibh.

(Muna mbeidh beo ina ndéidh ach seachtar)