Ruby, 137 134 130 bytes

Recursive function that returns the string. I'm trying to golf down the numeral encodings a bit more if possible, but I'm not sure how.

Whoops, it's practically a direct port of @Neil's ES6 answer now.

f=->x{(x<t=1e3)?"CM900D500CD400C100XC90L50XL40X10IX9V5IV4I1".gsub(/(\D+)(\d+)/){v=$2.to_i;s=x/v;x%=v;$1*s}:"(#{f[x/t]})#{f[x%t]}"}

Ruby, 137 134 130 129 bytes

Recursive function that returns the string. I'm trying to golf down the numeral encodings a bit more if possible, but I'm not sure how.

Whoops, it's practically a direct port of @Neil's ES6 answer now.

f=->x{(x<t=1e3)?"CM900D500CD400C100XC90L50XL40X10IX9V5IV4I1".gsub(/(\D+)(\d+)/){s=x/v=eval($2);x%=v;$1*s}:"(#{f[x/t]})#{f[x%t]}"}

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#Ruby, 137 134 130 bytes

Recursive function that returns the string. I'm trying to golf down the numeral encodings a bit more if possible, but I'm not sure how.

Whoops, it's practically a direct port of @Neil's ES6 answer now.

f=->x{(x<t=1e3)?"CM900D500CD400C100XC90L50XL40X10IX9V5IV4I1".gsub(/(\D+)(\d+)/){v=$2.to_i;s=x/v;x%=v;$1*s}:"(#{f[x/t]})#{f[x%t]}"}

Ruby, 137 134 130 bytes

Recursive function that returns the string. I'm trying to golf down the numeral encodings a bit more if possible, but I'm not sure how.

Whoops, it's practically a direct port of @Neil's ES6 answer now.

f=->x{(x<t=1e3)?"CM900D500CD400C100XC90L50XL40X10IX9V5IV4I1".gsub(/(\D+)(\d+)/){v=$2.to_i;s=x/v;x%=v;$1*s}:"(#{f[x/t]})#{f[x%t]}"}

#Ruby, 137 134 bytes

Preliminary solution, recursive function that returns the string. I'm trying to golf down the numeral encodings a bit more if possible, but I"m not sure how.

f=->x{s=(x<t=1e3)?"":"(#{f[x/t]})";x%=t
"CM900D500CD400C100XC90L50XL40X10IX9V5IV4I1".gsub(/(\D+)(\d+)/){v=$2.to_i;s+=$1*(x/v);x%=v};s}

#Ruby, 137 134 130 bytes

Recursive function that returns the string. I'm trying to golf down the numeral encodings a bit more if possible, but I'm not sure how.

Whoops, it's practically a direct port of @Neil's ES6 answer now.

f=->x{(x<t=1e3)?"CM900D500CD400C100XC90L50XL40X10IX9V5IV4I1".gsub(/(\D+)(\d+)/){v=$2.to_i;s=x/v;x%=v;$1*s}:"(#{f[x/t]})#{f[x%t]}"}