# data
x0 <- data.frame(d1 = c(10,20,30,4)
                 , n1 = c(10^3,10^4,10^3,10^2)
                 , d2 = c(20,200,20,5)
                 , n2 = c(10^3,10^4,10^3,10^2)
)
# Incidence rate ratio
x0$irr <- with(x0, d1/n1 / (d2/n2))
    # Standard errors of ln(irr)
    x0$ln_se <- with(x0,sqrt(1/d1+1/d2))
# Lower and upper CI
x0$cil <- round(with(x0,exp(log(irr) - 1.96*ln_se)),3)
    x0$ciu <- round(with(x0,exp(log(irr) + 1.96*ln_se)),3)
x0