If you know the Ensembl build that you used to get the Gene IDs, you can easily get the corresponding Gene Names from Biomart or even the GTF file of the same build. If you want to do it in R, use the biomaRt package.

Option 1: Using awk to get Gene ID and Name from GTF file:

You could use awk to get associated Gene ID & Gene Names from the GTF file:

awk '{                                
    for (i = 1; I <= NF; i++) {
        if ($i ~ /gene_id|gene_name/) {
            printf "%s ", $(i+1)
        }
    }
    print ""
}' Homo_sapiens.GRCh37.70.gtf | sed -e 's/"//g' -e 's/;//g' -e 's/ /\t/' | sort -k1,1 | uniq > Homo_sapiens.GRCh37.70.txt

Read this file in R

> annot = read.delim('Homo_sapiens.GRCh37.70.txt', header=F)

> head(annot)
               V1        V2
1 ENSG00000000003   TSPAN6 
2 ENSG00000000005     TNMD 
3 ENSG00000000419     DPM1 
4 ENSG00000000457    SCYL3 
5 ENSG00000000460 C1orf112 
6 ENSG00000000938      FGR

Merge your existing file with this annotation, assuming your file name is existingfile and the column containing Ensembl Gene IDs is GeneID

merged.file = merge(existingfile, annot, by.x='GeneID', by.y='V1')

Option 2: Using biomaRt in R:

library(biomaRt)
# Get an archived version of ensembl i.e. ensembl 70 in this case
ensembl = useMart("ENSEMBL_MART_ENSEMBL", dataset = "hsapiens_gene_ensembl", host = "jan2013.archive.ensembl.org")
# example list of ENSG ids
ensemblID = c('ENSG00000242959','ENSG00000160396','ENSG00000229494')
# use this list to get corresponding Gene Symbols
results = getBM(attributes = c('hgnc_symbol','ensembl_gene_id'), filters = "ensembl_gene_id", values = ensemblID, mart = ensembl)

# If you have a csv file you can read it like this
ensemblID = read.csv('file.csv')
# get the GeneIDs in the csv file
ensemblID = ensemblID[,1]
# use biomaRt
results = getBM(attributes = c('hgnc_symbol','ensembl_gene_id'), filters = "ensembl_gene_id", values = ensemblID, mart = ensembl)

Comparison of the two methods:

Using Option 1, you get this for the first three Ensembl IDs:

ENSG00000242959    RP4-599G15.3
ENSG00000160396    HIPK4
ENSG00000229494    AC012494.1

Using Option 2:

hgnc_symbol ensembl_gene_id
      HIPK4 ENSG00000160396
            ENSG00000229494
            ENSG00000242959

Therefore, I would recommend you use the first option.

You might be interested in the bioconductor package mygene.

Use BioMart. Here is a help video. Filter by ID list limit and your list of IDs, get the Ensembl ID and Associated Gene Name as attributes.